Intl Era Monthcode: Split out and expand end-of-month tests

These end-of-month tests were moved in from staging in
https://github.com/tc39/test262/pull/4667. Split them out into their own
files and expand to cover PlainDateTime and ZonedDateTime as well.

test/intl402/Temporal/PlainDate/prototype/add/basic-chinese.js

@@ -82,27 +82,12 @@ const months4 = new Temporal.Duration(0, 4);
 const months4n = new Temporal.Duration(0, -4);
 const months6 = new Temporal.Duration(0, 6);
 const months6n = new Temporal.Duration(0, -6);
-const durations = [
-  months1,
-  months1n,
-  months4,
-  months4n,
-  months6,
-  months6n,
-];
 
 const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
 const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
 const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
 const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
 const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
-const dates = [
-  date201901,
-  date201906,
-  date201911,
-  date201912,
-  date200012
-];
 
 TemporalHelpers.assertPlainDate(
   date201911.add(months1),
@@ -154,43 +139,6 @@ TemporalHelpers.assertPlainDate(
   2000, 6, "M06", 1, "Subtracting 6 months, with result in same year"
 );
 
-for (var duration of durations) {
-  for (var start of dates) {
-    const end = start.add(duration);
-
-    // startYesterday = start - (1 day)
-    const startYesterday = start.add({ days: -1 });
-    // endYesterday = startYesterday + duration
-    const endYesterday = startYesterday.add(duration);
-    // When adding months, the result day should be the same
-    // unless there are fewer days in the destination month than the source day
-    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
-
-    // endYesterdayNextDay = endYesterday + (1 day)
-    var endYesterdayNextDay = endYesterday.add({ days: 1 });
-    // Move forward to next first-day-of-month
-    while (endYesterdayNextDay.day !== 1) {
-      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
-    }
-
-    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
-
-    // endReverse should equal end
-    const endReverse = endYesterdayNextDay.add({ days: -1 });
-    const startReverse = endReverse.add(duration.negated());
-    // subtracting months give the same day unless there are fewer days in the destination month
-    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
-
-    // Move forward to next first-day-of-month
-    var startReverseNextDay = startReverse.add({ days: 1 });
-    while(startReverseNextDay.day !== 1) {
-      startReverseNextDay = startReverseNextDay.add({ days: 1 });
-    }
-
-    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
-  }
-}
-
 // Weeks
 
 const months2weeks3 = new Temporal.Duration(0, /* months = */ 2, /* weeks = */ 3);

test/intl402/Temporal/PlainDate/prototype/add/basic-dangi.js

@@ -82,27 +82,12 @@ const months4 = new Temporal.Duration(0, 4);
 const months4n = new Temporal.Duration(0, -4);
 const months6 = new Temporal.Duration(0, 6);
 const months6n = new Temporal.Duration(0, -6);
-const durations = [
-  months1,
-  months1n,
-  months4,
-  months4n,
-  months6,
-  months6n,
-];
 
 const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
 const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
 const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
 const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
 const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
-const dates = [
-  date201901,
-  date201906,
-  date201911,
-  date201912,
-  date200012
-];
 
 TemporalHelpers.assertPlainDate(
   date201911.add(months1),
@@ -154,43 +139,6 @@ TemporalHelpers.assertPlainDate(
   2000, 6, "M06", 1, "Subtracting 6 months, with result in same year"
 );
 
-for (var duration of durations) {
-  for (var start of dates) {
-    const end = start.add(duration);
-
-    // startYesterday = start - (1 day)
-    const startYesterday = start.add({ days: -1 });
-    // endYesterday = startYesterday + duration
-    const endYesterday = startYesterday.add(duration);
-    // When adding months, the result day should be the same
-    // unless there are fewer days in the destination month than the source day
-    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
-
-    // endYesterdayNextDay = endYesterday + (1 day)
-    var endYesterdayNextDay = endYesterday.add({ days: 1 });
-    // Move forward to next first-day-of-month
-    while (endYesterdayNextDay.day !== 1) {
-      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
-    }
-
-    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
-
-    // endReverse should equal end
-    const endReverse = endYesterdayNextDay.add({ days: -1 });
-    const startReverse = endReverse.add(duration.negated());
-    // subtracting months give the same day unless there are fewer days in the destination month
-    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
-
-    // Move forward to next first-day-of-month
-    var startReverseNextDay = startReverse.add({ days: 1 });
-    while(startReverseNextDay.day !== 1) {
-      startReverseNextDay = startReverseNextDay.add({ days: 1 });
-    }
-
-    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
-  }
-}
-
 // Weeks
 
 const months2weeks3 = new Temporal.Duration(0, /* months = */ 2, /* weeks = */ 3);

test/intl402/Temporal/PlainDate/prototype/add/month-boundary-chinese.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.add
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, 1);
+const months1n = new Temporal.Duration(0, -1);
+const months4 = new Temporal.Duration(0, 4);
+const months4n = new Temporal.Duration(0, -4);
+const months6 = new Temporal.Duration(0, 6);
+const months6n = new Temporal.Duration(0, -6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
+const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
+const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
+const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
+const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.add(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.add({ days: -1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.add(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.add({ days: 1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.add({ days: -1 });
+    const startReverse = endReverse.add(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.add({ days: 1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDate/prototype/add/month-boundary-dangi.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.add
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, 1);
+const months1n = new Temporal.Duration(0, -1);
+const months4 = new Temporal.Duration(0, 4);
+const months4n = new Temporal.Duration(0, -4);
+const months6 = new Temporal.Duration(0, 6);
+const months6n = new Temporal.Duration(0, -6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
+const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
+const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
+const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
+const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.add(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.add({ days: -1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.add(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.add({ days: 1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.add({ days: -1 });
+    const startReverse = endReverse.add(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.add({ days: 1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDate/prototype/subtract/basic-chinese.js

@@ -82,27 +82,12 @@ const months4 = new Temporal.Duration(0, -4);
 const months4n = new Temporal.Duration(0, 4);
 const months6 = new Temporal.Duration(0, -6);
 const months6n = new Temporal.Duration(0, 6);
-const durations = [
-  months1,
-  months1n,
-  months4,
-  months4n,
-  months6,
-  months6n,
-];
 
 const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
 const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
 const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
 const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
 const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
-const dates = [
-  date201901,
-  date201906,
-  date201911,
-  date201912,
-  date200012
-];
 
 TemporalHelpers.assertPlainDate(
   date201911.subtract(months1),
@@ -154,43 +139,6 @@ TemporalHelpers.assertPlainDate(
   2000, 6, "M06", 1, "Subtracting 6 months, with result in same year"
 );
 
-for (var duration of durations) {
-  for (var start of dates) {
-    const end = start.subtract(duration.negated());
-
-    // startYesterday = start - (1 day)
-    const startYesterday = start.subtract({ days: 1 });
-    // endYesterday = startYesterday + duration
-    const endYesterday = startYesterday.subtract(duration.negated());
-    // When adding months, the result day should be the same
-    // unless there are fewer days in the destination month than the source day
-    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
-
-    // endYesterdayNextDay = endYesterday + (1 day)
-    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
-    // Move forward to next first-day-of-month
-    while (endYesterdayNextDay.day !== 1) {
-      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
-    }
-
-    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
-
-    // endReverse should equal end
-    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
-    const startReverse = endReverse.subtract(duration);
-    // subtracting months give the same day unless there are fewer days in the destination month
-    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
-
-    // Move forward to next first-day-of-month
-    var startReverseNextDay = startReverse.subtract({ days: -1 });
-    while(startReverseNextDay.day !== 1) {
-      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
-    }
-
-    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
-  }
-}
-
 // Weeks
 
 const months2weeks3 = new Temporal.Duration(0, /* months = */ -2, /* weeks = */ -3);

test/intl402/Temporal/PlainDate/prototype/subtract/basic-dangi.js

@@ -82,27 +82,12 @@ const months4 = new Temporal.Duration(0, -4);
 const months4n = new Temporal.Duration(0, 4);
 const months6 = new Temporal.Duration(0, -6);
 const months6n = new Temporal.Duration(0, 6);
-const durations = [
-  months1,
-  months1n,
-  months4,
-  months4n,
-  months6,
-  months6n,
-];
 
 const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
 const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
 const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
 const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
 const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
-const dates = [
-  date201901,
-  date201906,
-  date201911,
-  date201912,
-  date200012
-];
 
 TemporalHelpers.assertPlainDate(
   date201911.subtract(months1),
@@ -154,43 +139,6 @@ TemporalHelpers.assertPlainDate(
   2000, 6, "M06", 1, "Subtracting 6 months, with result in same year"
 );
 
-for (var duration of durations) {
-  for (var start of dates) {
-    const end = start.subtract(duration);
-
-    // startYesterday = start - (1 day)
-    const startYesterday = start.subtract({ days: 1 });
-    // endYesterday = startYesterday + duration
-    const endYesterday = startYesterday.subtract(duration);
-    // When adding months, the result day should be the same
-    // unless there are fewer days in the destination month than the source day
-    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
-
-    // endYesterdayNextDay = endYesterday + (1 day)
-    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
-    // Move forward to next first-day-of-month
-    while (endYesterdayNextDay.day !== 1) {
-      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
-    }
-
-    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
-
-    // endReverse should equal end
-    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
-    const startReverse = endReverse.subtract(duration);
-    // subtracting months give the same day unless there are fewer days in the destination month
-    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
-
-    // Move forward to next first-day-of-month
-    var startReverseNextDay = startReverse.subtract({ days: -1 });
-    while(startReverseNextDay.day !== 1) {
-      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
-    }
-
-    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
-  }
-}
-
 // Weeks
 
 const months2weeks3 = new Temporal.Duration(0, /* months = */ -2, /* weeks = */ -3);

test/intl402/Temporal/PlainDate/prototype/subtract/month-boundary-chinese.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.subtract
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, -1);
+const months1n = new Temporal.Duration(0, 1);
+const months4 = new Temporal.Duration(0, -4);
+const months4n = new Temporal.Duration(0, 4);
+const months6 = new Temporal.Duration(0, -6);
+const months6n = new Temporal.Duration(0, 6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
+const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
+const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
+const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
+const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.subtract(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.subtract({ days: 1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.subtract(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
+    const startReverse = endReverse.subtract(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.subtract({ days: -1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDate/prototype/subtract/month-boundary-dangi.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.subtract
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, -1);
+const months1n = new Temporal.Duration(0, 1);
+const months4 = new Temporal.Duration(0, -4);
+const months4n = new Temporal.Duration(0, 4);
+const months6 = new Temporal.Duration(0, -6);
+const months6n = new Temporal.Duration(0, 6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDate.from({ year: 2019, monthCode: "M01", day: 1, calendar }, options);
+const date201906 = Temporal.PlainDate.from({ year: 2019, monthCode: "M06", day: 1, calendar }, options);
+const date201911 = Temporal.PlainDate.from({ year: 2019, monthCode: "M11", day: 1, calendar }, options);
+const date201912 = Temporal.PlainDate.from({ year: 2019, monthCode: "M12", day: 1, calendar }, options);
+const date200012 = Temporal.PlainDate.from({ year: 2000, monthCode: "M12", day: 1, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.subtract(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.subtract({ days: 1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.subtract(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDate(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
+    const startReverse = endReverse.subtract(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.subtract({ days: -1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDate(startReverseNextDay, start.year, start.month, start.monthCode, start.day, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDateTime/prototype/add/month-boundary-chinese.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.add
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, 1);
+const months1n = new Temporal.Duration(0, -1);
+const months4 = new Temporal.Duration(0, 4);
+const months4n = new Temporal.Duration(0, -4);
+const months6 = new Temporal.Duration(0, 6);
+const months6n = new Temporal.Duration(0, -6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201906 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201911 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201912 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const date200012 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.add(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.add({ days: -1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.add(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.add({ days: 1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.add({ days: -1 });
+    const startReverse = endReverse.add(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.add({ days: 1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay, start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDateTime/prototype/add/month-boundary-dangi.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.add
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, 1);
+const months1n = new Temporal.Duration(0, -1);
+const months4 = new Temporal.Duration(0, 4);
+const months4n = new Temporal.Duration(0, -4);
+const months6 = new Temporal.Duration(0, 6);
+const months6n = new Temporal.Duration(0, -6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201906 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201911 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201912 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const date200012 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.add(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.add({ days: -1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.add(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.add({ days: 1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.add({ days: -1 });
+    const startReverse = endReverse.add(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.add({ days: 1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay, start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDateTime/prototype/subtract/month-boundary-chinese.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.subtract
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, -1);
+const months1n = new Temporal.Duration(0, 1);
+const months4 = new Temporal.Duration(0, -4);
+const months4n = new Temporal.Duration(0, 4);
+const months6 = new Temporal.Duration(0, -6);
+const months6n = new Temporal.Duration(0, 6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201906 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201911 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201912 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const date200012 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.subtract(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.subtract({ days: 1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.subtract(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
+    const startReverse = endReverse.subtract(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.subtract({ days: -1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay, start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/PlainDateTime/prototype/subtract/month-boundary-dangi.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.subtract
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, -1);
+const months1n = new Temporal.Duration(0, 1);
+const months4 = new Temporal.Duration(0, -4);
+const months4n = new Temporal.Duration(0, 4);
+const months6 = new Temporal.Duration(0, -6);
+const months6n = new Temporal.Duration(0, 6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201906 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201911 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, calendar }, options);
+const date201912 = Temporal.PlainDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const date200012 = Temporal.PlainDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.subtract(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.subtract({ days: 1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.subtract(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay, end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
+    const startReverse = endReverse.subtract(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.subtract({ days: -1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay, start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/ZonedDateTime/prototype/add/month-boundary-chinese.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.add
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, 1);
+const months1n = new Temporal.Duration(0, -1);
+const months4 = new Temporal.Duration(0, 4);
+const months4n = new Temporal.Duration(0, -4);
+const months6 = new Temporal.Duration(0, 6);
+const months6n = new Temporal.Duration(0, -6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201906 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201911 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201912 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date200012 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.add(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.add({ days: -1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.add(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.add({ days: 1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay.toPlainDateTime(), end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.add({ days: -1 });
+    const startReverse = endReverse.add(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.add({ days: 1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay.toPlainDateTime(), start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/ZonedDateTime/prototype/add/month-boundary-dangi.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.add
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, 1);
+const months1n = new Temporal.Duration(0, -1);
+const months4 = new Temporal.Duration(0, 4);
+const months4n = new Temporal.Duration(0, -4);
+const months6 = new Temporal.Duration(0, 6);
+const months6n = new Temporal.Duration(0, -6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201906 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201911 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201912 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date200012 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.add(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.add({ days: -1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.add(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.add({ days: 1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay.toPlainDateTime(), end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.add({ days: -1 });
+    const startReverse = endReverse.add(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.add({ days: 1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.add({ days: 1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay.toPlainDateTime(), start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/ZonedDateTime/prototype/subtract/month-boundary-chinese.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.subtract
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "chinese";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, -1);
+const months1n = new Temporal.Duration(0, 1);
+const months4 = new Temporal.Duration(0, -4);
+const months4n = new Temporal.Duration(0, 4);
+const months6 = new Temporal.Duration(0, -6);
+const months6n = new Temporal.Duration(0, 6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201906 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201911 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201912 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date200012 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.subtract(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.subtract({ days: 1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.subtract(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay.toPlainDateTime(), end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
+    const startReverse = endReverse.subtract(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.subtract({ days: -1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay.toPlainDateTime(), start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}

test/intl402/Temporal/ZonedDateTime/prototype/subtract/month-boundary-dangi.js

@@ -0,0 +1,77 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.subtract
+description: Addition around end of month in the chinese calendar
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "dangi";
+const options = { overflow: "reject" };
+
+const months1 = new Temporal.Duration(0, -1);
+const months1n = new Temporal.Duration(0, 1);
+const months4 = new Temporal.Duration(0, -4);
+const months4n = new Temporal.Duration(0, 4);
+const months6 = new Temporal.Duration(0, -6);
+const months6n = new Temporal.Duration(0, 6);
+const durations = [
+  months1,
+  months1n,
+  months4,
+  months4n,
+  months6,
+  months6n,
+];
+
+const date201901 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M01", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201906 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M06", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201911 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M11", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date201912 = Temporal.ZonedDateTime.from({ year: 2019, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const date200012 = Temporal.ZonedDateTime.from({ year: 2000, monthCode: "M12", day: 1, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+const dates = [
+  date201901,
+  date201906,
+  date201911,
+  date201912,
+  date200012,
+];
+
+for (var duration of durations) {
+  for (var start of dates) {
+    const end = start.subtract(duration);
+
+    // startYesterday = start - (1 day)
+    const startYesterday = start.subtract({ days: 1 });
+    // endYesterday = startYesterday + duration
+    const endYesterday = startYesterday.subtract(duration);
+    // When adding months, the result day should be the same
+    // unless there are fewer days in the destination month than the source day
+    assert.sameValue(endYesterday.day, Math.min(startYesterday.day, endYesterday.daysInMonth), "adding months should result in same day");
+
+    // endYesterdayNextDay = endYesterday + (1 day)
+    var endYesterdayNextDay = endYesterday.subtract({ days: -1 });
+    // Move forward to next first-day-of-month
+    while (endYesterdayNextDay.day !== 1) {
+      endYesterdayNextDay = endYesterdayNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(endYesterdayNextDay.toPlainDateTime(), end.year, end.month, end.monthCode, end.day, 12, 34, 0, 0, 0, 0, `endYesterdayNextDay`, end.era, end.eraYear);
+
+    // endReverse should equal end
+    const endReverse = endYesterdayNextDay.subtract({ days: 1 });
+    const startReverse = endReverse.subtract(duration.negated());
+    // subtracting months give the same day unless there are fewer days in the destination month
+    assert.sameValue(startReverse.day, Math.min(endReverse.day, startReverse.daysInMonth));
+
+    // Move forward to next first-day-of-month
+    var startReverseNextDay = startReverse.subtract({ days: -1 });
+    while(startReverseNextDay.day !== 1) {
+      startReverseNextDay = startReverseNextDay.subtract({ days: -1 });
+    }
+
+    TemporalHelpers.assertPlainDateTime(startReverseNextDay.toPlainDateTime(), start.year, start.month, start.monthCode, start.day, 12, 34, 0, 0, 0, 0, `startReverseNextDay`, start.era, start.eraYear);
+  }
+}