Temporal: Wrapping at end of month, Coptic, Ethioaa, and Ethiopic calendars

Also correct some of the error message strings in the
wrapping-at-end-of-month-gregory tests

test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-coptic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.since
+description: Tests balancing of days to months at end of month (coptic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "coptic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pi Kogi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 28, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pi Kogi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 29, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pi Kogi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 30, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pi Kogi Enavot (M13)
+{
+  const end = Temporal.PlainDate.from({ year: 1971, monthCode: "M13", day: 6, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 6, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pi Kogi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 29, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pi Kogi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 30, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 5, calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pi Kogi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 6, calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pi Kogi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDate.from({ year: 1973, monthCode: "M13", day: 5, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 6, calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pi Kogi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 7, calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 29, calendar })
+      .since(Temporal.PlainDate.from({ year: 1970, monthCode: "M03", day: 28, calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 30, calendar })
+      .since(Temporal.PlainDate.from({ year: 1971, monthCode: "M05", day: 29, calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-ethioaa.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.since
+description: Tests balancing of days to months at end of month (ethioaa calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethioaa";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 28, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 29, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 30, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDate.from({ year: 1971, monthCode: "M13", day: 6, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 6, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 29, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 30, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 5, calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 6, calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDate.from({ year: 1973, monthCode: "M13", day: 5, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 6, calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 7, calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 29, calendar })
+      .since(Temporal.PlainDate.from({ year: 1970, monthCode: "M03", day: 28, calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 30, calendar })
+      .since(Temporal.PlainDate.from({ year: 1971, monthCode: "M05", day: 29, calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-ethiopic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.since
+description: Tests balancing of days to months at end of month (ethiopic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethiopic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 28, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 29, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 30, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDate.from({ year: 1971, monthCode: "M13", day: 6, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 6, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 29, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 30, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 5, calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 6, calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDate.from({ year: 1973, monthCode: "M13", day: 5, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 6, calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 7, calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 29, calendar })
+      .since(Temporal.PlainDate.from({ year: 1970, monthCode: "M03", day: 28, calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 30, calendar })
+      .since(Temporal.PlainDate.from({ year: 1971, monthCode: "M05", day: 29, calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-gregory.js

@@ -94,12 +94,12 @@ const calendar = "gregory";
   TemporalHelpers.assertDuration(
     Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 31, calendar }).since(end, { largestUnit: "months" }),
     0, -27, 0, -30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
   );
   TemporalHelpers.assertDuration(
     Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 31, calendar }).since(end, { largestUnit: "years" }),
     -2, -3, 0, -30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
   );
 }
 

test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-coptic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.until
+description: Tests balancing of days to months at end of month (coptic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "coptic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pi Kogi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 28, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pi Kogi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 29, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pi Kogi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 30, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pi Kogi Enavot (M13)
+{
+  const end = Temporal.PlainDate.from({ year: 1971, monthCode: "M13", day: 6, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 6, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pi Kogi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 29, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pi Kogi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 30, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 5, calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pi Kogi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 6, calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pi Kogi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDate.from({ year: 1973, monthCode: "M13", day: 5, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 6, calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pi Kogi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 7, calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 29, calendar })
+      .until(Temporal.PlainDate.from({ year: 1970, monthCode: "M03", day: 28, calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 30, calendar })
+      .until(Temporal.PlainDate.from({ year: 1971, monthCode: "M05", day: 29, calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-ethioaa.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.until
+description: Tests balancing of days to months at end of month (ethioaa calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethioaa";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 28, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 29, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 30, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDate.from({ year: 1971, monthCode: "M13", day: 6, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 6, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 29, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 30, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 5, calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 6, calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDate.from({ year: 1973, monthCode: "M13", day: 5, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 6, calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 7, calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 29, calendar })
+      .until(Temporal.PlainDate.from({ year: 1970, monthCode: "M03", day: 28, calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 30, calendar })
+      .until(Temporal.PlainDate.from({ year: 1971, monthCode: "M05", day: 29, calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-ethiopic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.until
+description: Tests balancing of days to months at end of month (ethiopic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethiopic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 28, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 29, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 30, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDate.from({ year: 1971, monthCode: "M13", day: 6, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 6, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 29, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1971, monthCode: "M12", day: 30, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDate.from({ year: 1970, monthCode: "M13", day: 5, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 5, calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDate.from({ year: 1970, monthCode: "M10", day: 6, calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDate.from({ year: 1973, monthCode: "M13", day: 5, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 5, calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 6, calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 7, calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 29, calendar })
+      .until(Temporal.PlainDate.from({ year: 1970, monthCode: "M03", day: 28, calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDate.from({ year: 1970, monthCode: "M01", day: 30, calendar })
+      .until(Temporal.PlainDate.from({ year: 1971, monthCode: "M05", day: 29, calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-gregory.js

@@ -94,12 +94,12 @@ const calendar = "gregory";
   TemporalHelpers.assertDuration(
     Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 31, calendar }).until(end, { largestUnit: "months" }),
     0, 27, 0, 30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
   );
   TemporalHelpers.assertDuration(
     Temporal.PlainDate.from({ year: 1970, monthCode: "M12", day: 31, calendar }).until(end, { largestUnit: "years" }),
     2, 3, 0, 30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
   );
 }
 

test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-coptic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: Tests balancing of days to months at end of month (coptic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "coptic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pi Kogi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pi Kogi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pi Kogi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pi Kogi Enavot (M13)
+{
+  const end = Temporal.PlainDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pi Kogi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pi Kogi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pi Kogi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pi Kogi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pi Kogi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, calendar })
+      .since(Temporal.PlainDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, calendar })
+      .since(Temporal.PlainDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-ethioaa.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: Tests balancing of days to months at end of month (ethioaa calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethioaa";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, calendar })
+      .since(Temporal.PlainDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, calendar })
+      .since(Temporal.PlainDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-ethiopic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: Tests balancing of days to months at end of month (ethiopic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethiopic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, calendar })
+      .since(Temporal.PlainDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, calendar })
+      .since(Temporal.PlainDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-gregory.js

@@ -94,12 +94,12 @@ const calendar = "gregory";
   TemporalHelpers.assertDuration(
     Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "months" }),
     0, -27, 0, -30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
   );
   TemporalHelpers.assertDuration(
     Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, calendar }).since(end, { largestUnit: "years" }),
     -2, -3, 0, -30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
   );
 }
 

test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-coptic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: Tests balancing of days to months at end of month (coptic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "coptic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pi Kogi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pi Kogi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pi Kogi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pi Kogi Enavot (M13)
+{
+  const end = Temporal.PlainDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pi Kogi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pi Kogi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pi Kogi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pi Kogi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pi Kogi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, calendar })
+      .until(Temporal.PlainDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, calendar })
+      .until(Temporal.PlainDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-ethioaa.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: Tests balancing of days to months at end of month (ethioaa calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethioaa";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, calendar })
+      .until(Temporal.PlainDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, calendar })
+      .until(Temporal.PlainDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-ethiopic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: Tests balancing of days to months at end of month (ethiopic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethiopic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.PlainDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.PlainDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.PlainDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.PlainDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, calendar })
+      .until(Temporal.PlainDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.PlainDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, calendar })
+      .until(Temporal.PlainDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-gregory.js

@@ -94,12 +94,12 @@ const calendar = "gregory";
   TemporalHelpers.assertDuration(
     Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "months" }),
     0, 27, 0, 30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
   );
   TemporalHelpers.assertDuration(
     Temporal.PlainDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, calendar }).until(end, { largestUnit: "years" }),
     2, 3, 0, 30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
   );
 }
 

test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-coptic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.since
+description: Tests balancing of days to months at end of month (coptic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "coptic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pi Kogi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pi Kogi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pi Kogi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pi Kogi Enavot (M13)
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pi Kogi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pi Kogi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pi Kogi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pi Kogi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pi Kogi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .since(Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .since(Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-ethioaa.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.since
+description: Tests balancing of days to months at end of month (ethioaa calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethioaa";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .since(Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .since(Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-ethiopic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.since
+description: Tests balancing of days to months at end of month (ethiopic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethiopic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, 0, 0, -6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit }),
+      0, -2, 0, -29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
+    0, -40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
+    -3, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
+    0, -39, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
+    -3, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .since(Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "months" }),
+    0, -1, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .since(Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "years" }),
+    -1, -3, 0, -29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-gregory.js

@@ -94,12 +94,12 @@ const calendar = "gregory";
   TemporalHelpers.assertDuration(
     Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "months" }),
     0, -27, 0, -30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
   );
   TemporalHelpers.assertDuration(
     Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, timeZone: "UTC", calendar }).since(end, { largestUnit: "years" }),
     -2, -3, 0, -30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
   );
 }
 

test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-coptic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.until
+description: Tests balancing of days to months at end of month (coptic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "coptic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pi Kogi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pi Kogi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pi Kogi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pi Kogi Enavot (M13)
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pi Kogi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pi Kogi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pi Kogi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pi Kogi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pi Kogi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pi Kogi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pi Kogi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .until(Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .until(Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-ethioaa.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.until
+description: Tests balancing of days to months at end of month (ethioaa calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethioaa";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .until(Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .until(Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-ethiopic.js

@@ -0,0 +1,121 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.until
+description: Tests balancing of days to months at end of month (ethiopic calendar)
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+const calendar = "ethiopic";
+
+// Difference between end of longer month to end of following shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 5th to Pikougi Enavot 5th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 28th to Pikougi Enavot 5th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 29th to Pikougi Enavot 5th is 6 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 5, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 5th is 5 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of leap-year Mesori (M12) to end of leap-year Pikougi Enavot (M13)
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Mesori 6th to Pikougi Enavot 6th is one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 7, 0, 0, 0, 0, 0, 0,
+      `Mesori 31st to Pikougi Enavot 6th is 7 days, not one month (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M12", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 0, 0, 6, 0, 0, 0, 0, 0, 0,
+      `Mesori 30th to Pikougi Enavot 6th is 6 days, not one month (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month to end of not-immediately-following
+// shorter month
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  for (const largestUnit of ["years", "months"]) {
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Paoni 5th to Pikougi Enavot 5th is 3 months (${largestUnit})`
+    );
+    TemporalHelpers.assertDuration(
+      Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M10", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit }),
+      0, 2, 0, 29, 0, 0, 0, 0, 0, 0,
+      `Paoni 6th to Pikougi Enavot 5th is 2 months 29 days, not 3 months (${largestUnit})`
+    );
+  }
+}
+
+// Difference between end of longer month in one year to shorter month in
+// later year
+{
+  const end = Temporal.ZonedDateTime.from({ year: 1973, monthCode: "M13", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar });
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
+    0, 40, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 5, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
+    3, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+    "Mesori 5th 1970 to Pikougi Enavot 5th 1973 is 3 years, 1 month"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
+    0, 39, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Mesori 6th 1970 to Pikougi Enavot 5th 1973 is 39 months, 29 days, not 40 months"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 7, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
+    3, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+    "Mesori 7th 1970 to Pikougi Enavot 5th 1973 is 3 years, 28 days"
+  );
+}
+
+// Difference where months passes through a month that's the same length or
+// shorter than either the start or end month
+{
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .until(Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M03", day: 28, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "months" }),
+    0, 1, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 29th to Hathor 28th is 1 month 29 days, not 59 days"
+  );
+  TemporalHelpers.assertDuration(
+    Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M01", day: 30, hour: 12, minute: 34, timeZone: "UTC", calendar })
+      .until(Temporal.ZonedDateTime.from({ year: 1971, monthCode: "M05", day: 29, hour: 12, minute: 34, timeZone: "UTC", calendar }), { largestUnit: "years" }),
+    1, 3, 0, 29, 0, 0, 0, 0, 0, 0,
+    "Thout 30th 1970 to Tobi 29th 1971 is 1 year, 3 months, 29 days, not 1 year, 2 months, 59 days"
+  );
+}

test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-gregory.js

@@ -94,12 +94,12 @@ const calendar = "gregory";
   TemporalHelpers.assertDuration(
     Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "months" }),
     0, 27, 0, 30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months"
   );
   TemporalHelpers.assertDuration(
     Temporal.ZonedDateTime.from({ year: 1970, monthCode: "M12", day: 31, hour: 12, minute: 34, timeZone: "UTC", calendar }).until(end, { largestUnit: "years" }),
     2, 3, 0, 30, 0, 0, 0, 0, 0, 0,
-    "Dec 30th 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
+    "Dec 31st 1970 to Apr 30th 1973 is 2 years, 3 months, 30 days, not 2 years 4 months"
   );
 }