diff --git a/test/intl402/Temporal/PlainDate/prototype/since/leap-months-chinese.js b/test/intl402/Temporal/PlainDate/prototype/since/leap-months-chinese.js
new file mode 100644
index 0000000000..f8d2911d78
--- /dev/null
+++ b/test/intl402/Temporal/PlainDate/prototype/since/leap-months-chinese.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in chinese calendar
+esid: sec-temporal.plaindate.prototype.since
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDate.from({ year: 2000, monthCode: "M04", day: 1, calendar }, options);
+const common1Month5 = Temporal.PlainDate.from({ year: 2000, monthCode: "M05", day: 1, calendar }, options);
+const common1Month6 = Temporal.PlainDate.from({ year: 2000, monthCode: "M06", day: 1, calendar }, options);
+const leapMonth4 = Temporal.PlainDate.from({ year: 2001, monthCode: "M04", day: 1, calendar }, options);
+const leapMonth4L = Temporal.PlainDate.from({ year: 2001, monthCode: "M04L", day: 1, calendar }, options);
+const leapMonth5 = Temporal.PlainDate.from({ year: 2001, monthCode: "M05", day: 1, calendar }, options);
+const common2Month4 = Temporal.PlainDate.from({ year: 2002, monthCode: "M04", day: 1, calendar }, options);
+const common2Month5 = Temporal.PlainDate.from({ year: 2002, monthCode: "M05", day: 1, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 common-leap backwards is -12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 leap-common backwards is -12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [-1, -1, 0, 0, "M04-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M04-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [-1, -1, 0, 0, "M04L-M05 backwards is -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M05 backwards is -13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 13, 0, 0, "M04-M04 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 12, 0, 0, "M04-M04 leap-common is 12mo not 13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 12, 0, 0, "M05-M05 common-leap is 12mo not 13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 13, 0, 0, "M05-M05 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, 12, 0, 0, "M04-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M04-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [1, 0, 0, 0, "M04L-M04 is 1y not 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M04 is 13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [1, 1, 0, 0, "M05-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M05-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, 12, 0, 0, "M04L-M05 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M05 is 12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.since(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.since(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.since(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.since(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.since(two);
+  const resultDaysISO = oneISO.since(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDate/prototype/since/leap-months-dangi.js b/test/intl402/Temporal/PlainDate/prototype/since/leap-months-dangi.js
new file mode 100644
index 0000000000..1bcd845c8f
--- /dev/null
+++ b/test/intl402/Temporal/PlainDate/prototype/since/leap-months-dangi.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in dangi calendar
+esid: sec-temporal.plaindate.prototype.since
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDate.from({ year: 2000, monthCode: "M04", day: 1, calendar }, options);
+const common1Month5 = Temporal.PlainDate.from({ year: 2000, monthCode: "M05", day: 1, calendar }, options);
+const common1Month6 = Temporal.PlainDate.from({ year: 2000, monthCode: "M06", day: 1, calendar }, options);
+const leapMonth4 = Temporal.PlainDate.from({ year: 2001, monthCode: "M04", day: 1, calendar }, options);
+const leapMonth4L = Temporal.PlainDate.from({ year: 2001, monthCode: "M04L", day: 1, calendar }, options);
+const leapMonth5 = Temporal.PlainDate.from({ year: 2001, monthCode: "M05", day: 1, calendar }, options);
+const common2Month4 = Temporal.PlainDate.from({ year: 2002, monthCode: "M04", day: 1, calendar }, options);
+const common2Month5 = Temporal.PlainDate.from({ year: 2002, monthCode: "M05", day: 1, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 common-leap backwards is -12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 leap-common backwards is -12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [-1, -1, 0, 0, "M04-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M04-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [-1, -1, 0, 0, "M04L-M05 backwards is -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M05 backwards is -13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 13, 0, 0, "M04-M04 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 12, 0, 0, "M04-M04 leap-common is 12mo not 13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 12, 0, 0, "M05-M05 common-leap is 12mo not 13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 13, 0, 0, "M05-M05 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, 12, 0, 0, "M04-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M04-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [1, 0, 0, 0, "M04L-M04 is 1y not 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M04 is 13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [1, 1, 0, 0, "M05-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M05-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, 12, 0, 0, "M04L-M05 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M05 is 12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.since(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.since(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.since(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.since(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.since(two);
+  const resultDaysISO = oneISO.since(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDate/prototype/until/leap-months-chinese.js b/test/intl402/Temporal/PlainDate/prototype/until/leap-months-chinese.js
new file mode 100644
index 0000000000..2838d3b576
--- /dev/null
+++ b/test/intl402/Temporal/PlainDate/prototype/until/leap-months-chinese.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in chinese calendar
+esid: sec-temporal.plaindate.prototype.until
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDate.from({ year: 2000, monthCode: "M04", day: 1, calendar }, options);
+const common1Month5 = Temporal.PlainDate.from({ year: 2000, monthCode: "M05", day: 1, calendar }, options);
+const common1Month6 = Temporal.PlainDate.from({ year: 2000, monthCode: "M06", day: 1, calendar }, options);
+const leapMonth4 = Temporal.PlainDate.from({ year: 2001, monthCode: "M04", day: 1, calendar }, options);
+const leapMonth4L = Temporal.PlainDate.from({ year: 2001, monthCode: "M04L", day: 1, calendar }, options);
+const leapMonth5 = Temporal.PlainDate.from({ year: 2001, monthCode: "M05", day: 1, calendar }, options);
+const common2Month4 = Temporal.PlainDate.from({ year: 2002, monthCode: "M04", day: 1, calendar }, options);
+const common2Month5 = Temporal.PlainDate.from({ year: 2002, monthCode: "M05", day: 1, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 12, 0, 0, "M04-M04 common-leap is 12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 13, 0, 0, "M04-M04 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 13, 0, 0, "M05-M05 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 12, 0, 0, "M05-M05 leap-common is 12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [1, 1, 0, 0, "M04-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M04-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, 12, 0, 0, "M04L-M04 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M04 is 12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, 12, 0, 0, "M05-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M05-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [1, 1, 0, 0, "M04L-M05 is 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M05 is 13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 leap-common backwards is -12mo not -13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 common-leap backwards is -12mo not -13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [-1, 0, 0, 0, "M04L-M04 backwards is -1y not -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M04 backwards is -13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [-1, -1, 0, 0, "M05-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M05-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.until(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.until(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.until(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.until(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.until(two);
+  const resultDaysISO = oneISO.until(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDate/prototype/until/leap-months-dangi.js b/test/intl402/Temporal/PlainDate/prototype/until/leap-months-dangi.js
new file mode 100644
index 0000000000..60722e0f35
--- /dev/null
+++ b/test/intl402/Temporal/PlainDate/prototype/until/leap-months-dangi.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in dangi calendar
+esid: sec-temporal.plaindate.prototype.until
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDate.from({ year: 2000, monthCode: "M04", day: 1, calendar }, options);
+const common1Month5 = Temporal.PlainDate.from({ year: 2000, monthCode: "M05", day: 1, calendar }, options);
+const common1Month6 = Temporal.PlainDate.from({ year: 2000, monthCode: "M06", day: 1, calendar }, options);
+const leapMonth4 = Temporal.PlainDate.from({ year: 2001, monthCode: "M04", day: 1, calendar }, options);
+const leapMonth4L = Temporal.PlainDate.from({ year: 2001, monthCode: "M04L", day: 1, calendar }, options);
+const leapMonth5 = Temporal.PlainDate.from({ year: 2001, monthCode: "M05", day: 1, calendar }, options);
+const common2Month4 = Temporal.PlainDate.from({ year: 2002, monthCode: "M04", day: 1, calendar }, options);
+const common2Month5 = Temporal.PlainDate.from({ year: 2002, monthCode: "M05", day: 1, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 12, 0, 0, "M04-M04 common-leap is 12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 13, 0, 0, "M04-M04 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 13, 0, 0, "M05-M05 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 12, 0, 0, "M05-M05 leap-common is 12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [1, 1, 0, 0, "M04-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M04-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, 12, 0, 0, "M04L-M04 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M04 is 12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, 12, 0, 0, "M05-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M05-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [1, 1, 0, 0, "M04L-M05 is 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M05 is 13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 leap-common backwards is -12mo not -13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 common-leap backwards is -12mo not -13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [-1, 0, 0, 0, "M04L-M04 backwards is -1y not -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M04 backwards is -13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [-1, -1, 0, 0, "M05-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M05-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.until(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.until(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.until(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.until(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.until(two);
+  const resultDaysISO = oneISO.until(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDate/prototype/until/until-across-lunisolar-leap-months.js b/test/intl402/Temporal/PlainDate/prototype/until/until-across-lunisolar-leap-months.js
deleted file mode 100644
index 10842ffcd2..0000000000
--- a/test/intl402/Temporal/PlainDate/prototype/until/until-across-lunisolar-leap-months.js
+++ /dev/null
@@ -1,20 +0,0 @@
-// Copyright (C) 2021 Igalia, S.L. All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-description: dateUntil works as expected after a leap month in a lunisolar calendar
-esid: sec-temporal.plaindate.prototype.until
-features: [Temporal, Intl.Era-monthcode]
----*/
-
-// 2001 is a leap year in the Chinese calendar with a M04L leap month.
-// Therefore, month: 6 is M05 in 2001 but M06 in 2000 which is not a leap year.
-const one = Temporal.PlainDate.from({ year: 2000, month: 6, day: 1, calendar: 'chinese' });
-const two = Temporal.PlainDate.from({ year: 2001, month: 6, day: 1, calendar: 'chinese' });
-
-const expected = { years: 'P12M', months: 'P12M', weeks: 'P50W4D', days: 'P354D' };
-
-Object.entries(expected).forEach(([largestUnit, expectedResult]) => {
-  const actualResult = one.until(two, { largestUnit });
-  assert.sameValue(actualResult.toString(), expectedResult);
-});
diff --git a/test/intl402/Temporal/PlainDateTime/prototype/since/leap-months-chinese.js b/test/intl402/Temporal/PlainDateTime/prototype/since/leap-months-chinese.js
new file mode 100644
index 0000000000..901ebe2dd7
--- /dev/null
+++ b/test/intl402/Temporal/PlainDateTime/prototype/since/leap-months-chinese.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in chinese calendar
+esid: sec-temporal.plaindatetime.prototype.since
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month5 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month6 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4L = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth5 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month4 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month5 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 common-leap backwards is -12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 leap-common backwards is -12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [-1, -1, 0, 0, "M04-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M04-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [-1, -1, 0, 0, "M04L-M05 backwards is -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M05 backwards is -13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 13, 0, 0, "M04-M04 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 12, 0, 0, "M04-M04 leap-common is 12mo not 13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 12, 0, 0, "M05-M05 common-leap is 12mo not 13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 13, 0, 0, "M05-M05 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, 12, 0, 0, "M04-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M04-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [1, 0, 0, 0, "M04L-M04 is 1y not 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M04 is 13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [1, 1, 0, 0, "M05-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M05-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, 12, 0, 0, "M04L-M05 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M05 is 12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.since(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.since(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.since(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.since(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.since(two);
+  const resultDaysISO = oneISO.since(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDateTime/prototype/since/leap-months-dangi.js b/test/intl402/Temporal/PlainDateTime/prototype/since/leap-months-dangi.js
new file mode 100644
index 0000000000..a29a57630e
--- /dev/null
+++ b/test/intl402/Temporal/PlainDateTime/prototype/since/leap-months-dangi.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in dangi calendar
+esid: sec-temporal.plaindatetime.prototype.since
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month5 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month6 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4L = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth5 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month4 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month5 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 common-leap backwards is -12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 leap-common backwards is -12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [-1, -1, 0, 0, "M04-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M04-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [-1, -1, 0, 0, "M04L-M05 backwards is -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M05 backwards is -13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 13, 0, 0, "M04-M04 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 12, 0, 0, "M04-M04 leap-common is 12mo not 13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 12, 0, 0, "M05-M05 common-leap is 12mo not 13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 13, 0, 0, "M05-M05 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, 12, 0, 0, "M04-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M04-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [1, 0, 0, 0, "M04L-M04 is 1y not 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M04 is 13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [1, 1, 0, 0, "M05-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M05-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, 12, 0, 0, "M04L-M05 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M05 is 12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.since(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.since(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.since(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.since(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.since(two);
+  const resultDaysISO = oneISO.since(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDateTime/prototype/until/leap-months-chinese.js b/test/intl402/Temporal/PlainDateTime/prototype/until/leap-months-chinese.js
new file mode 100644
index 0000000000..151b3aaf23
--- /dev/null
+++ b/test/intl402/Temporal/PlainDateTime/prototype/until/leap-months-chinese.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in chinese calendar
+esid: sec-temporal.plaindatetime.prototype.until
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month5 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month6 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4L = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth5 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month4 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month5 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 12, 0, 0, "M04-M04 common-leap is 12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 13, 0, 0, "M04-M04 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 13, 0, 0, "M05-M05 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 12, 0, 0, "M05-M05 leap-common is 12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [1, 1, 0, 0, "M04-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M04-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, 12, 0, 0, "M04L-M04 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M04 is 12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, 12, 0, 0, "M05-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M05-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [1, 1, 0, 0, "M04L-M05 is 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M05 is 13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 leap-common backwards is -12mo not -13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 common-leap backwards is -12mo not -13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [-1, 0, 0, 0, "M04L-M04 backwards is -1y not -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M04 backwards is -13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [-1, -1, 0, 0, "M05-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M05-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.until(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.until(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.until(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.until(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.until(two);
+  const resultDaysISO = oneISO.until(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/PlainDateTime/prototype/until/leap-months-dangi.js b/test/intl402/Temporal/PlainDateTime/prototype/until/leap-months-dangi.js
new file mode 100644
index 0000000000..8b2774e7a0
--- /dev/null
+++ b/test/intl402/Temporal/PlainDateTime/prototype/until/leap-months-dangi.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in dangi calendar
+esid: sec-temporal.plaindatetime.prototype.until
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month5 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common1Month6 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth4L = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, calendar }, options);
+const leapMonth5 = Temporal.PlainDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month4 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, calendar }, options);
+const common2Month5 = Temporal.PlainDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 12, 0, 0, "M04-M04 common-leap is 12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 13, 0, 0, "M04-M04 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 13, 0, 0, "M05-M05 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 12, 0, 0, "M05-M05 leap-common is 12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [1, 1, 0, 0, "M04-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M04-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, 12, 0, 0, "M04L-M04 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M04 is 12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, 12, 0, 0, "M05-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M05-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [1, 1, 0, 0, "M04L-M05 is 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M05 is 13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 leap-common backwards is -12mo not -13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 common-leap backwards is -12mo not -13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [-1, 0, 0, 0, "M04L-M04 backwards is -1y not -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M04 backwards is -13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [-1, -1, 0, 0, "M05-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M05-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.until(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.until(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.until(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.until(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.until(two);
+  const resultDaysISO = oneISO.until(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/ZonedDateTime/prototype/since/leap-months-chinese.js b/test/intl402/Temporal/ZonedDateTime/prototype/since/leap-months-chinese.js
new file mode 100644
index 0000000000..2fd2e15351
--- /dev/null
+++ b/test/intl402/Temporal/ZonedDateTime/prototype/since/leap-months-chinese.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in chinese calendar
+esid: sec-temporal.zoneddatetime.prototype.since
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month5 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month6 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4L = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth5 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month4 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month5 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 common-leap backwards is -12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 leap-common backwards is -12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [-1, -1, 0, 0, "M04-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M04-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [-1, -1, 0, 0, "M04L-M05 backwards is -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M05 backwards is -13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 13, 0, 0, "M04-M04 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 12, 0, 0, "M04-M04 leap-common is 12mo not 13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 12, 0, 0, "M05-M05 common-leap is 12mo not 13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 13, 0, 0, "M05-M05 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, 12, 0, 0, "M04-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M04-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [1, 0, 0, 0, "M04L-M04 is 1y not 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M04 is 13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [1, 1, 0, 0, "M05-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M05-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, 12, 0, 0, "M04L-M05 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M05 is 12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.since(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.since(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.since(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.since(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.since(two);
+  const resultDaysISO = oneISO.since(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/ZonedDateTime/prototype/since/leap-months-dangi.js b/test/intl402/Temporal/ZonedDateTime/prototype/since/leap-months-dangi.js
new file mode 100644
index 0000000000..659c1a5cfe
--- /dev/null
+++ b/test/intl402/Temporal/ZonedDateTime/prototype/since/leap-months-dangi.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in dangi calendar
+esid: sec-temporal.zoneddatetime.prototype.since
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month5 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month6 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4L = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth5 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month4 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month5 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 common-leap backwards is -12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 leap-common backwards is -12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [-1, -1, 0, 0, "M04-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M04-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M04 backwards is -12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M05-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [-1, -1, 0, 0, "M04L-M05 backwards is -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M05 backwards is -13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+    [0, -12, 0, 0, "M06-M05 common-leap backwards is -12mo not -11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 13, 0, 0, "M04-M04 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 12, 0, 0, "M04-M04 leap-common is 12mo not 13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 12, 0, 0, "M05-M05 common-leap is 12mo not 13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 13, 0, 0, "M05-M05 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, 12, 0, 0, "M04-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M04-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [1, 0, 0, 0, "M04L-M04 is 1y not 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M04 is 13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [1, 1, 0, 0, "M05-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M05-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, 12, 0, 0, "M04L-M05 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M05 is 12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.since(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.since(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.since(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.since(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.since(two);
+  const resultDaysISO = oneISO.since(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/ZonedDateTime/prototype/until/leap-months-chinese.js b/test/intl402/Temporal/ZonedDateTime/prototype/until/leap-months-chinese.js
new file mode 100644
index 0000000000..c29e5bfb18
--- /dev/null
+++ b/test/intl402/Temporal/ZonedDateTime/prototype/until/leap-months-chinese.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in chinese calendar
+esid: sec-temporal.zoneddatetime.prototype.until
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month5 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month6 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4L = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth5 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month4 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month5 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 12, 0, 0, "M04-M04 common-leap is 12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 13, 0, 0, "M04-M04 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 13, 0, 0, "M05-M05 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 12, 0, 0, "M05-M05 leap-common is 12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [1, 1, 0, 0, "M04-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M04-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, 12, 0, 0, "M04L-M04 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M04 is 12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, 12, 0, 0, "M05-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M05-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [1, 1, 0, 0, "M04L-M05 is 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M05 is 13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 leap-common backwards is -12mo not -13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 common-leap backwards is -12mo not -13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [-1, 0, 0, 0, "M04L-M04 backwards is -1y not -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M04 backwards is -13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [-1, -1, 0, 0, "M05-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M05-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.until(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.until(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.until(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.until(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.until(two);
+  const resultDaysISO = oneISO.until(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}
diff --git a/test/intl402/Temporal/ZonedDateTime/prototype/until/leap-months-dangi.js b/test/intl402/Temporal/ZonedDateTime/prototype/until/leap-months-dangi.js
new file mode 100644
index 0000000000..7a729b814b
--- /dev/null
+++ b/test/intl402/Temporal/ZonedDateTime/prototype/until/leap-months-dangi.js
@@ -0,0 +1,164 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Difference across leap months in dangi calendar
+esid: sec-temporal.zoneddatetime.prototype.until
+includes: [temporalHelpers.js]
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// 2001 is a leap year with a M04L leap month.
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const common1Month4 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month5 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common1Month6 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth4L = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M04L", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const leapMonth5 = Temporal.ZonedDateTime.from({ year: 2001, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month4 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M04", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const common2Month5 = Temporal.ZonedDateTime.from({ year: 2002, monthCode: "M05", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+// (receiver, argument, years test data, months test data)
+// test data: expected years, months, weeks, days, description
+// largestUnit years: make sure some cases where the answer is 12 months do not
+// balance up to 1 year
+// largestUnit months: similar to years, but make sure number of months in year
+// is computed correctly
+// For largestUnit of days and weeks, the results should be identical to what
+// the ISO calendar gives for the corresponding ISO dates
+const tests = [
+  [
+    common1Month4, leapMonth4,
+    [1, 0, 0, 0, "M04-M04 common-leap is 1y"],
+    [0, 12, 0, 0, "M04-M04 common-leap is 12mo"],
+  ],
+  [
+    leapMonth4, common2Month4,
+    [1, 0, 0, 0, "M04-M04 leap-common is 1y"],
+    [0, 13, 0, 0, "M04-M04 leap-common is 13mo not 12mo"],
+  ],
+  [
+    common1Month4, common2Month4,
+    [2, 0, 0, 0, "M04-M04 common-common is 2y"],
+    [0, 25, 0, 0, "M04-M04 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month5, leapMonth5,
+    [1, 0, 0, 0, "M05-M05 common-leap is 1y"],
+    [0, 13, 0, 0, "M05-M05 common-leap is 13mo not 12mo"],
+  ],
+  [
+    leapMonth5, common2Month5,
+    [1, 0, 0, 0, "M05-M05 leap-common is 1y"],
+    [0, 12, 0, 0, "M05-M05 leap-common is 12mo"],
+  ],
+  [
+    common1Month5, common2Month5,
+    [2, 0, 0, 0, "M05-M05 common-common is 2y"],
+    [0, 25, 0, 0, "M05-M05 common-common is 25mo not 24mo"],
+  ],
+  [
+    common1Month4, leapMonth4L,
+    [1, 1, 0, 0, "M04-M04L is 1y 1mo"],
+    [0, 13, 0, 0, "M04-M04L is 13mo"],
+  ],
+  [
+    leapMonth4L, common2Month4,
+    [0, 12, 0, 0, "M04L-M04 is 12mo not 1y"],
+    [0, 12, 0, 0, "M04L-M04 is 12mo"],
+  ],
+  [
+    common1Month5, leapMonth4L,
+    [0, 12, 0, 0, "M05-M04L is 12mo not 1y"],
+    [0, 12, 0, 0, "M05-M04L is 12mo"],
+  ],
+  [
+    leapMonth4L, common2Month5,
+    [1, 1, 0, 0, "M04L-M05 is 1y 1mo (exhibits calendar-specific constraining)"],
+    [0, 13, 0, 0, "M04L-M05 is 13mo"],
+  ],
+  [
+    common1Month6, leapMonth5,
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+    [0, 12, 0, 0, "M06-M05 common-leap is 12mo not 11mo"],
+  ],
+
+  // Negative
+  [
+    common2Month4, leapMonth4,
+    [-1, 0, 0, 0, "M04-M04 common-leap backwards is -1y"],
+    [0, -13, 0, 0, "M04-M04 common-leap backwards is -13mo not -12mo"],
+  ],
+  [
+    leapMonth4, common1Month4,
+    [-1, 0, 0, 0, "M04-M04 leap-common backwards is -1y"],
+    [0, -12, 0, 0, "M04-M04 leap-common backwards is -12mo not -13mo"],
+  ],
+  [
+    common2Month4, common1Month4,
+    [-2, 0, 0, 0, "M04-M04 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M04-M04 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month5, leapMonth5,
+    [-1, 0, 0, 0, "M05-M05 common-leap backwards is -1y"],
+    [0, -12, 0, 0, "M05-M05 common-leap backwards is -12mo not -13mo"],
+  ],
+  [
+    leapMonth5, common1Month5,
+    [-1, 0, 0, 0, "M05-M05 leap-common backwards is -1y"],
+    [0, -13, 0, 0, "M05-M05 leap-common backwards is -13mo not -12mo"],
+  ],
+  [
+    common2Month5, common1Month5,
+    [-2, 0, 0, 0, "M05-M05 common-common backwards is -2y"],
+    [0, -25, 0, 0, "M05-M05 common-common backwards is -25mo not -24mo"],
+  ],
+  [
+    common2Month4, leapMonth4L,
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04-M04L backwards is -12mo"],
+  ],
+  [
+    leapMonth4L, common1Month4,
+    [-1, 0, 0, 0, "M04L-M04 backwards is -1y not -1y -1mo (exhibits calendar-specific constraining)"],
+    [0, -13, 0, 0, "M04L-M04 backwards is -13mo"],
+  ],
+  [
+    common2Month5, leapMonth4L,
+    [-1, -1, 0, 0, "M05-M04L backwards is -1y -1mo"],
+    [0, -13, 0, 0, "M05-M04L backwards is -13mo"],
+  ],
+  [
+    leapMonth4L, common1Month5,
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo not -1y"],
+    [0, -12, 0, 0, "M04L-M05 backwards is -12mo"],
+  ],
+];
+
+for (const [one, two, yearsTest, monthsTest] of tests) {
+  let [years, months, weeks, days, descr] = yearsTest;
+  let result = one.until(two, { largestUnit: "years" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  [years, months, weeks, days, descr] = monthsTest;
+  result = one.until(two, { largestUnit: "months" });
+  TemporalHelpers.assertDuration(result, years, months, weeks, days, 0, 0, 0, 0, 0, 0, descr);
+
+  const oneISO = one.withCalendar("iso8601");
+  const twoISO = two.withCalendar("iso8601");
+
+  const resultWeeks = one.until(two, { largestUnit: "weeks" });
+  const resultWeeksISO = oneISO.until(twoISO, { largestUnit: "weeks" });
+  TemporalHelpers.assertDurationsEqual(resultWeeks, resultWeeksISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit weeks`);
+
+  const resultDays = one.until(two);
+  const resultDaysISO = oneISO.until(twoISO);
+  TemporalHelpers.assertDurationsEqual(resultDays, resultDaysISO,
+    `${one.year}-${one.monthCode}-${one.day} : ${two.year}-${two.monthCode}-${two.day} largestUnit days`);
+}