Temporal: Tests for corner case in TZDB involving DST transition in 1919

These tests add coverage for a corner case in the TZDB. In spring 1919,
the America/Toronto time zone switched to DST at 23:30 on March 30th,
skipping an hour ahead to 00:30 on March 31st. This meant that both March
30th and March 31st were 23.5-hour days.

See: https://github.com/tc39/proposal-temporal/issues/2910

test/intl402/Temporal/PlainDate/prototype/toZonedDateTime/dst-skipped-cross-midnight.js

@@ -0,0 +1,33 @@
+// 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.tozoneddatetime
+description: Test TZDB edge case where start of day is not 00:00 nor 01:00
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+// DST spring-forward hour skipped from 1919-03-30T23:30 to 1919-03-31T00:30, so
+// day starts at 00:30
+const instance = new Temporal.PlainDate(1919, 3, 31);
+const startOfDay = instance.toZonedDateTime("America/Toronto");
+const midnightDisambiguated = instance.toZonedDateTime({
+  timeZone: "America/Toronto",
+  plainTime: new Temporal.PlainTime(),
+});
+TemporalHelpers.assertDuration(
+  startOfDay.until(midnightDisambiguated),
+  0, 0, 0, 0, 0, /* minutes = */ 30, 0, 0, 0, 0,
+  "start of day is 30 minutes earlier than following the disambiguation strategy for midnight"
+);
+
+assert.sameValue(
+  startOfDay.epochNanoseconds,
+  instance.toZonedDateTime({ timeZone: "America/Toronto" }).epochNanoseconds,
+  "omitted plainTime is the same result as using the string shorthand"
+);
+assert.sameValue(
+  startOfDay.epochNanoseconds,
+  instance.toZonedDateTime({ timeZone: "America/Toronto", plainTime: undefined }).epochNanoseconds,
+  "explicitly undefined plainTime is the same result as using the string shorthand"
+);

test/intl402/Temporal/ZonedDateTime/from/dst-skipped-cross-midnight.js

@@ -0,0 +1,24 @@
+// 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.from
+description: Test TZDB edge case where start of day is not 00:00 nor 01:00
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+// DST spring-forward hour skipped from 1919-03-30T23:30 to 1919-03-31T00:30, so
+// day starts at 00:30
+const startOfDay = Temporal.ZonedDateTime.from("1919-03-31[America/Toronto]");
+const midnightDisambiguated = Temporal.ZonedDateTime.from("1919-03-31T00[America/Toronto]");
+TemporalHelpers.assertDuration(
+  startOfDay.until(midnightDisambiguated),
+  0, 0, 0, 0, 0, /* minutes = */ 30, 0, 0, 0, 0,
+  "start of day is 30 minutes earlier than following the disambiguation strategy for midnight"
+);
+
+assert.sameValue(
+  midnightDisambiguated.epochNanoseconds,
+  Temporal.ZonedDateTime.from({ year: 1919, month: 3, day: 31, timeZone: "America/Toronto" }).epochNanoseconds,
+  "start of day magic doesn't happen with property bag, missing properties are zero"
+);

test/intl402/Temporal/ZonedDateTime/prototype/hoursInDay/dst-skipped-cross-midnight.js

@@ -0,0 +1,27 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-get-temporal.zoneddatetime.prototype.hoursinday
+description: Test TZDB edge case where start of day is not 00:00 nor 01:00
+features: [Temporal]
+---*/
+
+// DST spring-forward hour skipped at 1919-03-30T23:30 (23.5 hour day)
+const dayBefore = Temporal.ZonedDateTime.from({
+  year: 1919,
+  month: 3,
+  day: 30,
+  hour: 12,
+  timeZone: "America/Toronto",
+});
+assert.sameValue(dayBefore.hoursInDay, 23.5, "1919-03-30 had 23.5 hours in America/Toronto");
+
+// Following day was also 23.5 hours
+const dayAfter = Temporal.ZonedDateTime.from({
+  year: 1919,
+  month: 3,
+  day: 31,
+  hour: 12,
+  timeZone: "America/Toronto",
+});
+assert.sameValue(dayAfter.hoursInDay, 23.5, "1919-03-31 had 23.5 hours in America/Toronto");

test/intl402/Temporal/ZonedDateTime/prototype/round/dst-skipped-cross-midnight.js

@@ -0,0 +1,49 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-get-temporal.zoneddatetime.prototype.round
+description: Test TZDB edge case where start of day is not 00:00 nor 01:00
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+// DST spring-forward hour skipped at 1919-03-30T23:30 (23.5 hour day)
+// 11.75 hours is 0.5
+const dayBefore = Temporal.ZonedDateTime.from({
+  year: 1919,
+  month: 3,
+  day: 30,
+  hour: 11,
+  minute: 45,
+  timeZone: "America/Toronto",
+});
+TemporalHelpers.assertPlainDateTime(
+  dayBefore.round({ smallestUnit: "day" }).toPlainDateTime(),
+  1919, 3, "M03", 31, 0, 30, 0, 0, 0, 0,
+  "1919-03-30T11:45 rounds up to start of next day with halfExpand"
+);
+TemporalHelpers.assertPlainDateTime(
+  dayBefore.round({ smallestUnit: "day", roundingMode: "halfTrunc" }).toPlainDateTime(),
+  1919, 3, "M03", 30, 0, 0, 0, 0, 0, 0,
+  "1919-03-30T11:45 rounds down to start of this day with halfTrunc"
+);
+
+// Following day was also 23.5 hours
+const dayAfter = Temporal.ZonedDateTime.from({
+  year: 1919,
+  month: 3,
+  day: 31,
+  hour: 12,
+  minute: 15,
+  timeZone: "America/Toronto",
+});
+TemporalHelpers.assertPlainDateTime(
+  dayAfter.round({ smallestUnit: "day" }).toPlainDateTime(),
+  1919, 4, "M04", 1, 0, 0, 0, 0, 0, 0,
+  "1919-03-31T12:15 rounds up to start of next day with halfExpand"
+);
+TemporalHelpers.assertPlainDateTime(
+  dayAfter.round({ smallestUnit: "day", roundingMode: "halfTrunc" }).toPlainDateTime(),
+  1919, 3, "M03", 31, 0, 30, 0, 0, 0, 0,
+  "1919-03-31T12:15 rounds down to start of this day with halfTrunc"
+);

test/intl402/Temporal/ZonedDateTime/prototype/startOfDay/dst-skipped-cross-midnight.js

@@ -0,0 +1,20 @@
+// 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.startofday
+description: Test TZDB edge case where start of day is not 00:00 nor 01:00
+features: [Temporal]
+---*/
+
+// DST spring-forward hour skipped at 1919-03-30T23:30, so the following day
+// started at 00:30
+const instance = Temporal.ZonedDateTime.from({
+  year: 1919,
+  month: 3,
+  day: 31,
+  hour: 12,
+  timeZone: "America/Toronto",
+});
+const result = instance.startOfDay();
+assert.sameValue(result.hour, 0, "1919-03-31 started at hour 0");
+assert.sameValue(result.minute, 30, "1919-03-31 started at minute 30");

test/intl402/Temporal/ZonedDateTime/prototype/withPlainTime/dst-skipped-cross-midnight.js

@@ -0,0 +1,25 @@
+// 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.withplaintime
+description: Test TZDB edge case where start of day is not 00:00 nor 01:00
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+// DST spring-forward hour skipped from 1919-03-30T23:30 to 1919-03-31T00:30, so
+// day starts at 00:30
+const instance = Temporal.ZonedDateTime.from({
+  year: 1919,
+  month: 3,
+  day: 31,
+  hour: 12,
+  timeZone: "America/Toronto",
+});
+const startOfDay = instance.withPlainTime();
+const midnightDisambiguated = instance.withPlainTime(new Temporal.PlainTime());
+TemporalHelpers.assertDuration(
+  startOfDay.until(midnightDisambiguated),
+  0, 0, 0, 0, 0, /* minutes = */ 30, 0, 0, 0, 0,
+  "start of day is 30 minutes earlier than following the disambiguation strategy for midnight"
+);