From 60faf4f2bb05d8bd08d70feabdfcf878cbc485a9 Mon Sep 17 00:00:00 2001
From: Philip Chimento <pchimento@igalia.com>
Date: Mon, 15 Sep 2025 17:15:10 -0700
Subject: [PATCH] Temporal: Add tests covering end-of-month behaviour for
 ISO-like non-ISO calendars

Same tests as added in #4004, but for the four non-ISO calendars which
use ISO-like months and days.
---
 .../wrapping-at-end-of-month-gregorian.js     | 122 +++++++++++++++
 .../wrapping-at-end-of-month-gregorian.js     | 122 +++++++++++++++
 .../wrapping-at-end-of-month-gregorian.js     | 146 ++++++++++++++++++
 .../wrapping-at-end-of-month-gregorian.js     | 146 ++++++++++++++++++
 .../wrapping-at-end-of-month-gregorian.js     | 122 +++++++++++++++
 .../wrapping-at-end-of-month-gregorian.js     | 120 ++++++++++++++
 6 files changed, 778 insertions(+)
 create mode 100644 test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-gregorian.js
 create mode 100644 test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-gregorian.js
 create mode 100644 test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js
 create mode 100644 test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js
 create mode 100644 test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js
 create mode 100644 test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js

diff --git a/test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-gregorian.js b/test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-gregorian.js
new file mode 100644
index 0000000000..b4585cf2f9
--- /dev/null
+++ b/test/intl402/Temporal/PlainDate/prototype/since/wrapping-at-end-of-month-gregorian.js
@@ -0,0 +1,122 @@
+// Copyright (C) 2024 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 (ISO-like non-ISO calendars)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+for (const calendar of ["buddhist", "gregory", "japanese", "roc"]) {
+  // Difference between end of longer month to end of following shorter month
+  {
+    const end = new Temporal.PlainDate(1970, 2, 28, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 28, calendar).since(end, { largestUnit }),
+        0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 28th to Feb 28th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 29, calendar).since(end, { largestUnit }),
+        0, 0, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 28th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 30, calendar).since(end, { largestUnit }),
+        0, 0, 0, -29, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 28th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 31, calendar).since(end, { largestUnit }),
+        0, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 28th is 28 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of leap-year January to end of leap-year February
+  {
+    const end = new Temporal.PlainDate(1972, 2, 29, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1972, 1, 29, calendar).since(end, { largestUnit }),
+        0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 29th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1972, 1, 30, calendar).since(end, { largestUnit }),
+        0, 0, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 29th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1972, 1, 31, calendar).since(end, { largestUnit }),
+        0, 0, 0, -29, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 29th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month to end of not-immediately-following
+  // shorter month
+  {
+    const end = new Temporal.PlainDate(1970, 11, 30, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 8, 30, calendar).since(end, { largestUnit }),
+        0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Aug 30th to Nov 30th is 3 months (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 8, 31, calendar).since(end, { largestUnit }),
+        0, -2, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Aug 31st to Nov 30th is 2 months 30 days, not 3 months (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month in one year to shorter month in
+  // later year
+  {
+    const end = new Temporal.PlainDate(1973, 4, 30, calendar);
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 30, calendar).since(end, { largestUnit: "months" }),
+      0, -28, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 30, calendar).since(end, { largestUnit: "years" }),
+      -2, -4, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 2 years, 4 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 31, calendar).since(end, { largestUnit: "months" }),
+      0, -27, 0, -30, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 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 (${calendar})`
+    );
+  }
+
+  // Difference where months passes through a month that's the same length or
+  // shorter than either the start or end month
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 1, 29, calendar)
+        .since(new Temporal.PlainDate(1970, 3, 28, calendar), { largestUnit: "months" }),
+      0, -1, 0, -28, 0, 0, 0, 0, 0, 0,
+      `Jan 29th to Mar 28th is 1 month 28 days, not 58 days (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 1, 31, calendar)
+        .since(new Temporal.PlainDate(1971, 5, 30, calendar), { largestUnit: "years" }),
+      -1, -3, 0, -30, 0, 0, 0, 0, 0, 0,
+      `Jan 31st 1970 to May 30th 1971 is 1 year, 3 months, 30 days, not 1 year, 2 months, 60 days (${calendar})`
+    );
+  }
+}
diff --git a/test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-gregorian.js b/test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-gregorian.js
new file mode 100644
index 0000000000..166a58516a
--- /dev/null
+++ b/test/intl402/Temporal/PlainDate/prototype/until/wrapping-at-end-of-month-gregorian.js
@@ -0,0 +1,122 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.until
+description: >
+  Tests balancing of days to months at end of month (ISO-like non-ISO calendars)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+for (const calendar of ["buddhist", "gregory", "japanese", "roc"]) {
+  // Difference between end of longer month to end of following shorter month
+  {
+    const end = new Temporal.PlainDate(1970, 2, 28, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 28, calendar).until(end, { largestUnit }),
+        0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 28th to Feb 28th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 29, calendar).until(end, { largestUnit }),
+        0, 0, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 28th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 30, calendar).until(end, { largestUnit }),
+        0, 0, 0, 29, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 28th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 1, 31, calendar).until(end, { largestUnit }),
+        0, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 28th is 28 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of leap-year January to end of leap-year February
+  {
+    const end = new Temporal.PlainDate(1972, 2, 29, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1972, 1, 29, calendar).until(end, { largestUnit }),
+        0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 29th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1972, 1, 30, calendar).until(end, { largestUnit }),
+        0, 0, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 29th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1972, 1, 31, calendar).until(end, { largestUnit }),
+        0, 0, 0, 29, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 29th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month to end of not-immediately-following
+  // shorter month
+  {
+    const end = new Temporal.PlainDate(1970, 11, 30, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 8, 30, calendar).until(end, { largestUnit }),
+        0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Aug 30th to Nov 30th is 3 months (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDate(1970, 8, 31, calendar).until(end, { largestUnit }),
+        0, 2, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Aug 31st to Nov 30th is 2 months 30 days, not 3 months (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month in one year to shorter month in
+  // later year
+  {
+    const end = new Temporal.PlainDate(1973, 4, 30, calendar);
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 30, calendar).until(end, { largestUnit: "months" }),
+      0, 28, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 30, calendar).until(end, { largestUnit: "years" }),
+      2, 4, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 2 years, 4 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 31, calendar).until(end, { largestUnit: "months" }),
+      0, 27, 0, 30, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 12, 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 (${calendar})`
+    );
+  }
+
+  // Difference where months passes through a month that's the same length or
+  // shorter than either the start or end month
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 1, 29, calendar)
+        .until(new Temporal.PlainDate(1970, 3, 28, calendar), { largestUnit: "months" }),
+      0, 1, 0, 28, 0, 0, 0, 0, 0, 0,
+      `Jan 29th to Mar 28th is 1 month 28 days, not 58 days (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDate(1970, 1, 31, calendar)
+        .until(new Temporal.PlainDate(1971, 5, 30, calendar), { largestUnit: "years" }),
+      1, 3, 0, 30, 0, 0, 0, 0, 0, 0,
+      `Jan 31st 1970 to May 30th 1971 is 1 year, 3 months, 30 days, not 1 year, 2 months, 60 days (${calendar})`
+    );
+  }
+}
diff --git a/test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js b/test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js
new file mode 100644
index 0000000000..6206937489
--- /dev/null
+++ b/test/intl402/Temporal/PlainDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js
@@ -0,0 +1,146 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: >
+  Tests balancing of days to months at end of month (ISO-like non-ISO calendars)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+for (const calendar of ["buddhist", "gregory", "japanese", "roc"]) {
+  // Difference between end of longer month to end of following shorter month
+  {
+    const end = new Temporal.PlainDateTime(1970, 2, 28, 0, 0, 0, 0, 0, 0, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 28, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 28th to Feb 28th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 29, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, 0, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 28th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 30, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, 0, 0, -29, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 28th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 31, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 28th is 28 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of leap-year January to end of leap-year February
+  {
+    const end = new Temporal.PlainDateTime(1972, 2, 29, 0, 0, 0, 0, 0, 0, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1972, 1, 29, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 29th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1972, 1, 30, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, 0, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 29th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1972, 1, 31, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, 0, 0, -29, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 29th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month to end of not-immediately-following
+  // shorter month
+  {
+    const end = new Temporal.PlainDateTime(1970, 11, 30, 0, 0, 0, 0, 0, 0, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 8, 30, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Aug 30th to Nov 30th is 3 months (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 8, 31, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit }),
+        0, -2, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Aug 31st to Nov 30th is 2 months 30 days, not 3 months (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month in one year to shorter month in
+  // later year
+  {
+    const end = new Temporal.PlainDateTime(1973, 4, 30, 0, 0, 0, 0, 0, 0, calendar);
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 30, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit: "months" }),
+      0, -28, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 30, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit: "years" }),
+      -2, -4, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 2 years, 4 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 31, 0, 0, 0, 0, 0, 0, calendar).since(end, { largestUnit: "months" }),
+      0, -27, 0, -30, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 31, 0, 0, 0, 0, 0, 0, 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 (${calendar})`
+    );
+  }
+
+  // Difference where months passes through a month that's the same length or
+  // shorter than either the start or end month
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 1, 29, 0, 0, 0, 0, 0, 0, calendar)
+        .since(new Temporal.PlainDateTime(1970, 3, 28, 0, 0, 0, 0, 0, 0, calendar), { largestUnit: "months" }),
+      0, -1, 0, -28, 0, 0, 0, 0, 0, 0,
+      `Jan 29th to Mar 28th is 1 month 28 days, not 58 days (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 1, 31, 0, 0, 0, 0, 0, 0, calendar)
+        .since(new Temporal.PlainDateTime(1971, 5, 30, 0, 0, 0, 0, 0, 0, calendar), { largestUnit: "years" }),
+      -1, -3, 0, -30, 0, 0, 0, 0, 0, 0,
+      `Jan 31st 1970 to May 30th 1971 is 1 year, 3 months, 30 days, not 1 year, 2 months, 60 days (${calendar})`
+    );
+  }
+
+  // Test that 1-day backoff to maintain date/time sign compatibility backs-off from correct end
+  // while moving *forwards* in time and does not interfere with month boundaries
+  // https://github.com/tc39/proposal-temporal/issues/2820
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(2023, 2, 28, 3, 0, 0, 0, 0, 0, calendar)
+        .since(new Temporal.PlainDateTime(2023, 4, 1, 2, 0, 0, 0, 0, 0, calendar), { largestUnit: "years" }),
+      0, -1, 0, -3, -23, 0, 0, 0, 0, 0,
+      `Feb 28th (3am) to Apr 1st (2am) is -1 month, -3 days, and -23 hours (${calendar})`
+    );
+  }
+
+  // Test that 1-day backoff to maintain date/time sign compatibility backs-off from correct end
+  // while moving *backwards* in time and does not interfere with month boundaries
+  // https://github.com/tc39/proposal-temporal/issues/2820
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(2023, 3, 1, 2, 0, 0, 0, 0, 0, calendar)
+        .since(new Temporal.PlainDateTime(2023, 1, 1, 3, 0, 0, 0, 0, 0, calendar), { largestUnit: "years" }),
+      0, 1, 0, 30, 23, 0, 0, 0, 0, 0,
+      `Mar 1st (2am) to Jan 1st (3am) is 1 month, 30 days, and 23 hours (${calendar})`
+    );
+  }
+}
diff --git a/test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js b/test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js
new file mode 100644
index 0000000000..8b4663bd1d
--- /dev/null
+++ b/test/intl402/Temporal/PlainDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js
@@ -0,0 +1,146 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: >
+  Tests balancing of days to months at end of month (ISO-like non-ISO calendars)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+for (const calendar of ["buddhist", "gregory", "japanese", "roc"]) {
+  // Difference between end of longer month to end of following shorter month
+  {
+    const end = new Temporal.PlainDateTime(1970, 2, 28, 0, 0, 0, 0, 0, 0, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 28, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 28th to Feb 28th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 29, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 0, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 28th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 30, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 0, 0, 29, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 28th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 1, 31, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 28th is 28 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of leap-year January to end of leap-year February
+  {
+    const end = new Temporal.PlainDateTime(1972, 2, 29, 0, 0, 0, 0, 0, 0, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1972, 1, 29, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 29th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1972, 1, 30, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 0, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 29th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1972, 1, 31, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 0, 0, 29, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 29th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month to end of not-immediately-following
+  // shorter month
+  {
+    const end = new Temporal.PlainDateTime(1970, 11, 30, 0, 0, 0, 0, 0, 0, calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 8, 30, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Aug 30th to Nov 30th is 3 months (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.PlainDateTime(1970, 8, 31, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit }),
+        0, 2, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Aug 31st to Nov 30th is 2 months 30 days, not 3 months (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month in one year to shorter month in
+  // later year
+  {
+    const end = new Temporal.PlainDateTime(1973, 4, 30, 0, 0, 0, 0, 0, 0, calendar);
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 30, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit: "months" }),
+      0, 28, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 30, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit: "years" }),
+      2, 4, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 2 years, 4 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 31, 0, 0, 0, 0, 0, 0, calendar).until(end, { largestUnit: "months" }),
+      0, 27, 0, 30, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 12, 31, 0, 0, 0, 0, 0, 0, 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 (${calendar})`
+    );
+  }
+
+  // Difference where months passes through a month that's the same length or
+  // shorter than either the start or end month
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 1, 29, 0, 0, 0, 0, 0, 0, calendar)
+        .until(new Temporal.PlainDateTime(1970, 3, 28, 0, 0, 0, 0, 0, 0, calendar), { largestUnit: "months" }),
+      0, 1, 0, 28, 0, 0, 0, 0, 0, 0,
+      `Jan 29th to Mar 28th is 1 month 28 days, not 58 days (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(1970, 1, 31, 0, 0, 0, 0, 0, 0, calendar)
+        .until(new Temporal.PlainDateTime(1971, 5, 30, 0, 0, 0, 0, 0, 0, calendar), { largestUnit: "years" }),
+      1, 3, 0, 30, 0, 0, 0, 0, 0, 0,
+      `Jan 31st 1970 to May 30th 1971 is 1 year, 3 months, 30 days, not 1 year, 2 months, 60 days (${calendar})`
+    );
+  }
+
+  // Test that 1-day backoff to maintain date/time sign compatibility backs-off from correct end
+  // while moving *backwards* in time and does not interfere with month boundaries
+  // https://github.com/tc39/proposal-temporal/issues/2820
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(2023, 2, 28, 3, 0, 0, 0, 0, 0, calendar)
+        .until(new Temporal.PlainDateTime(2023, 4, 1, 2, 0, 0, 0, 0, 0, calendar), { largestUnit: "years" }),
+      0, 1, 0, 3, 23, 0, 0, 0, 0, 0,
+      `Feb 28th (3am) to Apr 1st (2am) is 1 month, 3 days, and 23 hours (${calendar})`
+    );
+  }
+
+  // Test that 1-day backoff to maintain date/time sign compatibility backs-off from correct end
+  // while moving *forwards* in time and does not interfere with month boundaries
+  // https://github.com/tc39/proposal-temporal/issues/2820
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.PlainDateTime(2023, 3, 1, 2, 0, 0, 0, 0, 0, calendar)
+        .until(new Temporal.PlainDateTime(2023, 1, 1, 3, 0, 0, 0, 0, 0, calendar), { largestUnit: "years" }),
+      0, -1, 0, -30, -23, 0, 0, 0, 0, 0,
+      `Mar 1st (2am) to Jan 1st (3am) is -1 month, -30 days, and -23 hours (${calendar})`
+    );
+  }
+}
diff --git a/test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js b/test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js
new file mode 100644
index 0000000000..9c41728900
--- /dev/null
+++ b/test/intl402/Temporal/ZonedDateTime/prototype/since/wrapping-at-end-of-month-gregorian.js
@@ -0,0 +1,122 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.since
+description: >
+  Tests balancing of days to months at end of month (ISO-like non-ISO calendars)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+for (const calendar of ["buddhist", "gregory", "japanese", "roc"]) {
+  // Difference between end of longer month to end of following shorter month
+  {
+    const end = new Temporal.ZonedDateTime(5011200_000_000_000n /* = 1970-02-28T00Z */, "UTC", calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2332800_000_000_000n /* = 1970-01-28T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 28th to Feb 28th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2419200_000_000_000n /* = 1970-01-29T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, 0, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 28th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2505600_000_000_000n /* = 1970-01-30T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, 0, 0, -29, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 28th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2592000_000_000_000n /* = 1970-01-31T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, 0, 0, -28, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 28th is 28 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of leap-year January to end of leap-year February
+  {
+    const end = new Temporal.ZonedDateTime(68169600_000_000_000n /* = 1972-02-29T00Z */, "UTC", calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(65491200_000_000_000n /* = 1972-01-29T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, -1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 29th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(65577600_000_000_000n /* = 1972-01-30T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, 0, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 29th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(65664000_000_000_000n /* = 1972-01-31T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, 0, 0, -29, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 29th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month to end of not-immediately-following
+  // shorter month
+  {
+    const end = new Temporal.ZonedDateTime(28771200_000_000_000n /* = 1970-11-30T00Z */, "UTC", calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(20822400_000_000_000n /* = 1970-08-30T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, -3, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Aug 30th to Nov 30th is 3 months (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(20908800_000_000_000n /* = 1970-08-31T00Z */, "UTC", calendar).since(end, { largestUnit }),
+        0, -2, 0, -30, 0, 0, 0, 0, 0, 0,
+        `Aug 31st to Nov 30th is 2 months 30 days, not 3 months (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month in one year to shorter month in
+  // later year
+  {
+    const end = new Temporal.ZonedDateTime(104976000_000_000_000n /* = 1973-04-30T00Z */, "UTC", calendar);
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31363200_000_000_000n /* = 1970-12-30T00Z */, "UTC", calendar).since(end, { largestUnit: "months" }),
+      0, -28, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31363200_000_000_000n /* = 1970-12-30T00Z */, "UTC", calendar).since(end, { largestUnit: "years" }),
+      -2, -4, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 2 years, 4 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31449600_000_000_000n /* = 1970-12-31T00Z */, "UTC", calendar).since(end, { largestUnit: "months" }),
+      0, -27, 0, -30, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31449600_000_000_000n /* = 1970-12-31T00Z */, "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 (${calendar})`
+    );
+  }
+
+  // Difference where months passes through a month that's the same length or
+  // shorter than either the start or end month
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(2419200_000_000_000n /* = 1970-01-29T00Z */, "UTC", calendar)
+        .since(new Temporal.ZonedDateTime(7430400_000_000_000n /* = 1970-03-28T00Z */, "UTC", calendar), { largestUnit: "months" }),
+      0, -1, 0, -28, 0, 0, 0, 0, 0, 0,
+      `Jan 29th to Mar 28th is 1 month 28 days, not 58 days (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(2592000_000_000_000n /* = 1970-01-31T00Z */, "UTC", calendar)
+        .since(new Temporal.ZonedDateTime(44409600_000_000_000n /* = 1971-05-30T00Z */, "UTC", calendar), { largestUnit: "years" }),
+      -1, -3, 0, -30, 0, 0, 0, 0, 0, 0,
+      `Jan 31st 1970 to May 30th 1971 is 1 year, 3 months, 30 days, not 1 year, 2 months, 60 days (${calendar})`
+    );
+  }
+}
diff --git a/test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js b/test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js
new file mode 100644
index 0000000000..5607102b70
--- /dev/null
+++ b/test/intl402/Temporal/ZonedDateTime/prototype/until/wrapping-at-end-of-month-gregorian.js
@@ -0,0 +1,120 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.until
+description: >
+  Tests balancing of days to months at end of month (ISO-like non-ISO calendars)
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+for (const calendar of ["buddhist", "gregory", "japanese", "roc"]) {
+  // Difference between end of longer month to end of following shorter month
+  {
+    const end = new Temporal.ZonedDateTime(5011200_000_000_000n /* = 1970-02-28T00Z */, "UTC", calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2332800_000_000_000n /* = 1970-01-28T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 28th to Feb 28th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2419200_000_000_000n /* = 1970-01-29T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 0, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 28th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2505600_000_000_000n /* = 1970-01-30T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 0, 0, 29, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 28th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(2592000_000_000_000n /* = 1970-01-31T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 0, 0, 28, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 28th is 28 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of leap-year January to end of leap-year February
+  {
+    const end = new Temporal.ZonedDateTime(68169600_000_000_000n /* = 1972-02-29T00Z */, "UTC", calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(65491200_000_000_000n /* = 1972-01-29T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 1, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Jan 29th to Feb 29th is one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(65577600_000_000_000n /* = 1972-01-30T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 0, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Jan 30th to Feb 29th is 30 days, not one month (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(65664000_000_000_000n /* = 1972-01-31T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 0, 0, 29, 0, 0, 0, 0, 0, 0,
+        `Jan 31st to Feb 29th is 29 days, not one month (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month to end of not-immediately-following
+  // shorter month
+  {
+    const end = new Temporal.ZonedDateTime(28771200_000_000_000n /* = 1970-11-30T00Z */, "UTC", calendar);
+    for (const largestUnit of ["years", "months"]) {
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(20822400_000_000_000n /* = 1970-08-30T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 3, 0, 0, 0, 0, 0, 0, 0, 0,
+        `Aug 30th to Nov 30th is 3 months (${calendar}, ${largestUnit})`
+      );
+      TemporalHelpers.assertDuration(
+        new Temporal.ZonedDateTime(20908800_000_000_000n /* = 1970-08-31T00Z */, "UTC", calendar).until(end, { largestUnit }),
+        0, 2, 0, 30, 0, 0, 0, 0, 0, 0,
+        `Aug 31st to Nov 30th is 2 months 30 days, not 3 months (${calendar}, ${largestUnit})`
+      );
+    }
+  }
+
+  // Difference between end of longer month in one year to shorter month in
+  // later year
+  {
+    const end = new Temporal.ZonedDateTime(104976000_000_000_000n /* = 1973-04-30T00Z */, "UTC", calendar);
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31363200_000_000_000n /* = 1970-12-30T00Z */, "UTC", calendar).until(end, { largestUnit: "months" }),
+      0, 28, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31363200_000_000_000n /* = 1970-12-30T00Z */, "UTC", calendar).until(end, { largestUnit: "years" }),
+      2, 4, 0, 0, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 2 years, 4 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31449600_000_000_000n /* = 1970-12-31T00Z */, "UTC", calendar).until(end, { largestUnit: "months" }),
+      0, 27, 0, 30, 0, 0, 0, 0, 0, 0,
+      `Dec 30th 1970 to Apr 30th 1973 is 27 months, 30 days, not 28 months (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(31449600_000_000_000n /* = 1970-12-31T00Z */, "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 (${calendar})`
+    );
+  }
+
+  // Difference where months passes through a month that's the same length or
+  // shorter than either the start or end month
+  {
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(2419200_000_000_000n /* = 1970-01-29T00Z */, "UTC", calendar).until(new Temporal.ZonedDateTime(7430400_000_000_000n /* = 1970-03-28T00Z */, "UTC", calendar), { largestUnit: "months" }),
+      0, 1, 0, 28, 0, 0, 0, 0, 0, 0,
+      `Jan 29th to Mar 28th is 1 month 28 days, not 58 days (${calendar})`
+    );
+    TemporalHelpers.assertDuration(
+      new Temporal.ZonedDateTime(2592000_000_000_000n /* = 1970-01-31T00Z */, "UTC", calendar).until(new Temporal.ZonedDateTime(44409600_000_000_000n /* = 1971-05-30T00Z */, "UTC", calendar), { largestUnit: "years" }),
+      1, 3, 0, 30, 0, 0, 0, 0, 0, 0,
+      `Jan 31st 1970 to May 30th 1971 is 1 year, 3 months, 30 days, not 1 year, 2 months, 60 days (${calendar})`
+    );
+  }
+}