Intl Era Monthcode: Addition on leap day in Orthodox calendars

This adds tests for addition and subtraction on leap day (M13-06) and
constraining to M13-05 in common years for the three Orthodox calendars
(coptic, ethioaa, ethiopic).

test/intl402/Temporal/PlainDate/prototype/add/leap-year-coptic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (coptic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "coptic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDate.from({ year: 1739, monthCode: "M13", day: 6, calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years1),
+  1740, 13, "M13", 5, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 1740);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years1n),
+  1738, 13, "M13", 5, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 1738);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years4, options),
+  1743, 13, "M13", 6, "Adding 4 years to leap day goes to the next leap day",
+  "am", 1743);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years4n, options),
+  1735, 13, "M13", 6, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 1735);

test/intl402/Temporal/PlainDate/prototype/add/leap-year-ethioaa.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethioaa calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethioaa";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDate.from({ year: 7515, monthCode: "M13", day: 6, calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years1),
+  7516, 13, "M13", 5, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "aa", 7516);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years1n),
+  7514, 13, "M13", 5, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "aa", 7514);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years4, options),
+  7519, 13, "M13", 6, "Adding 4 years to leap day goes to the next leap day",
+  "aa", 7519);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years4n, options),
+  7511, 13, "M13", 6, "Subtracting 4 years from leap day goes to the previous leap day",
+  "aa", 7511);

test/intl402/Temporal/PlainDate/prototype/add/leap-year-ethiopic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethiopic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethiopic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDate.from({ year: 2015, monthCode: "M13", day: 6, calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years1),
+  2016, 13, "M13", 5, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 2016);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years1n),
+  2014, 13, "M13", 5, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 2014);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years4, options),
+  2019, 13, "M13", 6, "Adding 4 years to leap day goes to the next leap day",
+  "am", 2019);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.add(years4n, options),
+  2011, 13, "M13", 6, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 2011);

test/intl402/Temporal/PlainDate/prototype/subtract/leap-year-coptic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (coptic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "coptic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDate.from({ year: 1739, monthCode: "M13", day: 6, calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years1),
+  1740, 13, "M13", 5, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 1740);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years1n),
+  1738, 13, "M13", 5, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 1738);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years4, options),
+  1743, 13, "M13", 6, "Adding 4 years to leap day goes to the next leap day",
+  "am", 1743);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years4n, options),
+  1735, 13, "M13", 6, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 1735);

test/intl402/Temporal/PlainDate/prototype/subtract/leap-year-ethioaa.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethioaa calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethioaa";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDate.from({ year: 7515, monthCode: "M13", day: 6, calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years1),
+  7516, 13, "M13", 5, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "aa", 7516);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years1n),
+  7514, 13, "M13", 5, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "aa", 7514);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years4, options),
+  7519, 13, "M13", 6, "Adding 4 years to leap day goes to the next leap day",
+  "aa", 7519);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years4n, options),
+  7511, 13, "M13", 6, "Subtracting 4 years from leap day goes to the previous leap day",
+  "aa", 7511);

test/intl402/Temporal/PlainDate/prototype/subtract/leap-year-ethiopic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethiopic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethiopic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDate.from({ year: 2015, monthCode: "M13", day: 6, calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years1),
+  2016, 13, "M13", 5, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 2016);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years1n),
+  2014, 13, "M13", 5, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 2014);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years4, options),
+  2019, 13, "M13", 6, "Adding 4 years to leap day goes to the next leap day",
+  "am", 2019);
+
+TemporalHelpers.assertPlainDate(
+  leapDay.subtract(years4n, options),
+  2011, 13, "M13", 6, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 2011);

test/intl402/Temporal/PlainDateTime/prototype/add/leap-year-coptic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (coptic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "coptic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDateTime.from({ year: 1739, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1),
+  1740, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 1740);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1n),
+  1738, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 1738);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4, options),
+  1743, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 1743);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4n, options),
+  1735, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 1735);

test/intl402/Temporal/PlainDateTime/prototype/add/leap-year-ethioaa.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethioaa calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethioaa";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDateTime.from({ year: 7515, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1),
+  7516, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "aa", 7516);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1n),
+  7514, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "aa", 7514);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4, options),
+  7519, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "aa", 7519);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4n, options),
+  7511, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "aa", 7511);

test/intl402/Temporal/PlainDateTime/prototype/add/leap-year-ethiopic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethiopic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethiopic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDateTime.from({ year: 2015, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1),
+  2016, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 2016);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1n),
+  2014, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 2014);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4, options),
+  2019, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 2019);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4n, options),
+  2011, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 2011);

test/intl402/Temporal/PlainDateTime/prototype/subtract/leap-year-coptic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (coptic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "coptic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDateTime.from({ year: 1739, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1),
+  1740, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 1740);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1n),
+  1738, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 1738);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4, options),
+  1743, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 1743);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4n, options),
+  1735, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 1735);

test/intl402/Temporal/PlainDateTime/prototype/subtract/leap-year-ethioaa.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethioaa calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethioaa";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDateTime.from({ year: 7515, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1),
+  7516, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "aa", 7516);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1n),
+  7514, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "aa", 7514);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4, options),
+  7519, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "aa", 7519);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4n, options),
+  7511, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "aa", 7511);

test/intl402/Temporal/PlainDateTime/prototype/subtract/leap-year-ethiopic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethiopic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethiopic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.PlainDateTime.from({ year: 2015, monthCode: "M13", day: 6, hour: 12, minute: 34, calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1),
+  2016, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 2016);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1n),
+  2014, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 2014);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4, options),
+  2019, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 2019);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4n, options),
+  2011, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 2011);

test/intl402/Temporal/ZonedDateTime/prototype/add/leap-year-coptic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (coptic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "coptic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.ZonedDateTime.from({ year: 1739, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1).toPlainDateTime(),
+  1740, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 1740);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1n).toPlainDateTime(),
+  1738, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 1738);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4, options).toPlainDateTime(),
+  1743, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 1743);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4n, options).toPlainDateTime(),
+  1735, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 1735);

test/intl402/Temporal/ZonedDateTime/prototype/add/leap-year-ethioaa.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethioaa calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethioaa";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.ZonedDateTime.from({ year: 7515, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1).toPlainDateTime(),
+  7516, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "aa", 7516);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1n).toPlainDateTime(),
+  7514, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "aa", 7514);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4, options).toPlainDateTime(),
+  7519, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "aa", 7519);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4n, options).toPlainDateTime(),
+  7511, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "aa", 7511);

test/intl402/Temporal/ZonedDateTime/prototype/add/leap-year-ethiopic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethiopic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethiopic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.ZonedDateTime.from({ year: 2015, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+const years1 = new Temporal.Duration(1);
+const years1n = new Temporal.Duration(-1);
+const years4 = new Temporal.Duration(4);
+const years4n = new Temporal.Duration(-4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1).toPlainDateTime(),
+  2016, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 2016);
+assert.throws(RangeError, function () {
+  leapDay.add(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years1n).toPlainDateTime(),
+  2014, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 2014);
+assert.throws(RangeError, function () {
+  leapDay.add(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4, options).toPlainDateTime(),
+  2019, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 2019);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.add(years4n, options).toPlainDateTime(),
+  2011, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 2011);

test/intl402/Temporal/ZonedDateTime/prototype/subtract/leap-year-coptic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (coptic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "coptic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.ZonedDateTime.from({ year: 1739, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1).toPlainDateTime(),
+  1740, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 1740);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1n).toPlainDateTime(),
+  1738, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 1738);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4, options).toPlainDateTime(),
+  1743, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 1743);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4n, options).toPlainDateTime(),
+  1735, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 1735);

test/intl402/Temporal/ZonedDateTime/prototype/subtract/leap-year-ethioaa.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethioaa calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethioaa";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.ZonedDateTime.from({ year: 7515, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1).toPlainDateTime(),
+  7516, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "aa", 7516);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1n).toPlainDateTime(),
+  7514, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "aa", 7514);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4, options).toPlainDateTime(),
+  7519, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "aa", 7519);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4n, options).toPlainDateTime(),
+  7511, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "aa", 7511);

test/intl402/Temporal/ZonedDateTime/prototype/subtract/leap-year-ethiopic.js

@@ -0,0 +1,45 @@
+// 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: Check various basic calculations involving leap years (ethiopic calendar)
+features: [Temporal, Intl.Era-monthcode]
+includes: [temporalHelpers.js]
+---*/
+
+const calendar = "ethiopic";
+const options = { overflow: "reject" };
+
+const leapDay = Temporal.ZonedDateTime.from({ year: 2015, monthCode: "M13", day: 6, hour: 12, minute: 34, timeZone: "UTC", calendar }, options);
+
+const years1 = new Temporal.Duration(-1);
+const years1n = new Temporal.Duration(1);
+const years4 = new Temporal.Duration(-4);
+const years4n = new Temporal.Duration(4);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1).toPlainDateTime(),
+  2016, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Adding 1 year to leap day constrains to day 5 of epagomenal month",
+  "am", 2016);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1, options);
+}, "Adding 1 year to leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years1n).toPlainDateTime(),
+  2014, 13, "M13", 5, 12, 34, 0, 0, 0, 0, "Subtracting 1 year from leap day constrains to day 5 of epagomenal month",
+  "am", 2014);
+assert.throws(RangeError, function () {
+  leapDay.subtract(years1n, options);
+}, "Subtracting 1 year from leap day rejects");
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4, options).toPlainDateTime(),
+  2019, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Adding 4 years to leap day goes to the next leap day",
+  "am", 2019);
+
+TemporalHelpers.assertPlainDateTime(
+  leapDay.subtract(years4n, options).toPlainDateTime(),
+  2011, 13, "M13", 6, 12, 34, 0, 0, 0, 0, "Subtracting 4 years from leap day goes to the previous leap day",
+  "am", 2011);