[Temporal] Formatting Plain* Temporal objects should ignore the timezones.

These are tests for proposal-temporal's normative change
PR tc39/proposal-temporal#3246 and
polyfill fix PR tc39/proposal-temporal#3257

Author: ioannad

test/intl402/DateTimeFormat/prototype/format/temporal-objects-ignore-timezone.js

@@ -0,0 +1,52 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-datetime-format-functions
+description: Intl.DateTimeFormat.prototype.format ignores timezone when isPlain is true.
+features: [Temporal]
+---*/
+
+// Non existent date in the Pacific/Apia timezone.
+const datetime_apia = '2011-12-30T12:00:00';
+// Non existent time in the America/Los_Angeles timezone.
+const datetime_los_angeles = '2026-03-08T02:00:00';
+
+const pdt_apia = Temporal.PlainDateTime.from(datetime_apia);
+const pdt_los_angeles = Temporal.PlainDateTime.from(datetime_los_angeles);
+
+const pd_apia = Temporal.PlainDate.from(datetime_apia);
+const pd_los_angeles = Temporal.PlainDate.from(datetime_los_angeles);
+
+const pt_apia = Temporal.PlainTime.from(datetime_apia);
+const pt_los_angeles = Temporal.PlainTime.from(datetime_los_angeles);
+
+const dtf_apia = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'Pacific/Apia' });
+const dtf_los_angeles = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'America/Los_Angeles' });
+
+// PlainDateTime
+assert.sameValue(
+  dtf_apia.format(pdt_apia),
+  "12/30/11, 12:00 PM",
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+assert.sameValue(
+  dtf_los_angeles.format(pdt_los_angeles),
+  "3/8/26, 2:00 AM",
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);
+
+// PlainDate
+assert.sameValue(
+  dtf_apia.format(pd_apia),
+  "12/30/11",
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+// PlainTime
+assert.sameValue(
+  dtf_los_angeles.format(pt_los_angeles),
+  "2:00 AM",
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);

test/intl402/DateTimeFormat/prototype/formatRange/temporal-objects-ignore-timezone.js

@@ -0,0 +1,56 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-datetime-format-functions
+description: Intl.DateTimeFormat.prototype.formatRange ignores timezone when isPlain is true.
+features: [Temporal]
+---*/
+
+// Non existent date in the Pacific/Apia timezone.
+const datetime_apia = '2011-12-30T12:00:00';
+// Non existent time in the America/Los_Angeles timezone.
+const datetime_los_angeles = '2026-03-08T02:00:00';
+
+const pdt_apia = Temporal.PlainDateTime.from(datetime_apia);
+const pdt_los_angeles = Temporal.PlainDateTime.from(datetime_los_angeles);
+
+const pd_apia = Temporal.PlainDate.from(datetime_apia);
+const pd_los_angeles = Temporal.PlainDate.from(datetime_los_angeles);
+
+const pt_apia = Temporal.PlainTime.from(datetime_apia);
+const pt_los_angeles = Temporal.PlainTime.from(datetime_los_angeles);
+
+const dtf_apia = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'Pacific/Apia' });
+const dtf_los_angeles = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'America/Los_Angeles' });
+
+const usDateRangeSeparator = new Intl.DateTimeFormat("en-US", { dateStyle: "short" })
+  .formatRangeToParts(1 * 86400 * 1000, 366 * 86400 * 1000)
+  .find((part) => part.type === "literal" && part.source === "shared").value;
+
+// PlainDateTime
+assert.sameValue(
+  dtf_apia.formatRange(pdt_apia, pdt_los_angeles),
+  `12/30/11, 12:00 PM${usDateRangeSeparator}3/8/26, 2:00 AM`,
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+assert.sameValue(
+  dtf_los_angeles.formatRange(pdt_apia, pdt_los_angeles),
+  `12/30/11, 12:00 PM${usDateRangeSeparator}3/8/26, 2:00 AM`,
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);
+
+// PlainDate
+assert.sameValue(
+  dtf_apia.formatRange(pd_apia, pd_los_angeles),
+  `12/30/11${usDateRangeSeparator}3/8/26`,
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+// PlainTime
+assert.sameValue(
+  dtf_los_angeles.formatRange(pt_apia, pt_los_angeles),
+  `12:00 PM${usDateRangeSeparator}2:00 AM`,
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);

test/intl402/DateTimeFormat/prototype/formatRangeToParts/temporal-objects-ignore-timezone.js

@@ -0,0 +1,52 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-datetime-format-functions
+description: Intl.DateTimeFormat.prototype.formatRangeToParts ignores timezone when isPlain is true.
+features: [Temporal]
+---*/
+
+// Non existent date in the Pacific/Apia timezone.
+const datetime_apia = '2011-12-30T12:00:00';
+// Non existent time in the America/Los_Angeles timezone.
+const datetime_los_angeles = '2026-03-08T02:00:00';
+
+const pdt_apia = Temporal.PlainDateTime.from(datetime_apia);
+const pdt_los_angeles = Temporal.PlainDateTime.from(datetime_los_angeles);
+
+const pd_apia = Temporal.PlainDate.from(datetime_apia);
+const pd_los_angeles = Temporal.PlainDate.from(datetime_los_angeles);
+
+const pt_apia = Temporal.PlainTime.from(datetime_apia);
+const pt_los_angeles = Temporal.PlainTime.from(datetime_los_angeles);
+
+const dtf_apia = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'Pacific/Apia' });
+const dtf_los_angeles = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'America/Los_Angeles' });
+
+// PlainDateTime
+assert.sameValue(
+  dtf_apia.formatRangeToParts(pdt_apia, pdt_los_angeles)[2].value,
+  "30",
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+assert.sameValue(
+  dtf_los_angeles.formatRangeToParts(pdt_apia, pdt_los_angeles)[18].value,
+  "2",
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);
+
+// PlainDate
+assert.sameValue(
+  dtf_apia.formatRangeToParts(pd_apia, pd_los_angeles)[2].value,
+  "30",
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+// PlainTime
+assert.sameValue(
+  dtf_los_angeles.formatRangeToParts(pt_apia, pt_los_angeles)[6].value,
+  "2",
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);

test/intl402/DateTimeFormat/prototype/formatToParts/temporal-objects-ignore-timezone.js

@@ -0,0 +1,52 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-datetime-format-functions
+description: Intl.DateTimeFormat.prototype.formatToParts ignores timezone when isPlain is true.
+features: [Temporal]
+---*/
+
+// Non existent date in the Pacific/Apia timezone.
+const datetime_apia = '2011-12-30T12:00:00';
+// Non existent time in the America/Los_Angeles timezone.
+const datetime_los_angeles = '2026-03-08T02:00:00';
+
+const pdt_apia = Temporal.PlainDateTime.from(datetime_apia);
+const pdt_los_angeles = Temporal.PlainDateTime.from(datetime_los_angeles);
+
+const pd_apia = Temporal.PlainDate.from(datetime_apia);
+const pd_los_angeles = Temporal.PlainDate.from(datetime_los_angeles);
+
+const pt_apia = Temporal.PlainTime.from(datetime_apia);
+const pt_los_angeles = Temporal.PlainTime.from(datetime_los_angeles);
+
+const dtf_apia = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'Pacific/Apia' });
+const dtf_los_angeles = new Intl.DateTimeFormat('en-US', { dateStyle: 'short', timeStyle: 'short', timeZone: 'America/Los_Angeles' });
+
+// PlainDateTime
+assert.sameValue(
+  dtf_apia.formatToParts(pdt_apia)[2].value,
+  "30",
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+assert.sameValue(
+  dtf_los_angeles.formatToParts(pdt_los_angeles)[6].value,
+  "2",
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);
+
+// PlainDate
+assert.sameValue(
+  dtf_apia.formatToParts(pd_apia)[2].value,
+  "30",
+  "day is calculated correctly, ignoring the Pacific/Apia timezone"
+);
+
+// PlainTime
+assert.sameValue(
+  dtf_los_angeles.formatToParts(pt_los_angeles)[0].value,
+  "2",
+  "hour is calculated correctly with the America/Los_Angeles timezone"
+);

test/intl402/Temporal/Instant/prototype/toLocaleString/respect-timezone-after-formatting-plaindatetime.js

@@ -0,0 +1,37 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.tolocalestring
+description: Formatting a Plain* object should not break the formatter for Instant.
+features: [Temporal]
+---*/
+
+const datestring = '2026-03-29T02:30:15+01:00';
+const instant = Temporal.Instant.from(datestring);
+const pdt = Temporal.PlainDateTime.from(datestring);
+
+assert.sameValue(
+  pdt.toLocaleString('en', { timeStyle: 'long' }),
+  '2:30:15 AM'
+);
+
+assert.sameValue(
+  instant.toLocaleString('en', { timeStyle: 'long', timeZone: 'America/Los_Angeles' }),
+  '6:30:15 PM PDT'
+);
+
+assert.sameValue(
+  instant.toLocaleString('en', { timeStyle: 'long', timeZone: 'Europe/Berlin' }),
+  '3:30:15 AM GMT+2'
+);
+
+assert.sameValue(
+  new Temporal.PlainDate(2011, 12, 30).toLocaleString('en'),
+  '12/30/2011'
+),
+
+assert.sameValue(
+  new Temporal.Instant(0n).toLocaleString("en", { era: "narrow" }),
+  new Date(0).toLocaleString(["en"], { era: "narrow" })
+);

test/intl402/Temporal/PlainDate/prototype/toLocaleString/ignore-timezone.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.tolocalestring
+description: Temporal.PlainDate should be interpreted and formatted as wall-clock time
+features: [Temporal]
+---*/
+
+// A non-existent date in the Pacific/Apia timezone.
+const instance = Temporal.PlainDate.from({ year: 2011, month: 12, day: 30 });
+
+assert.sameValue(
+  instance.toLocaleString('en-US', { timeZone: 'Pacific/Apia' }),
+  '12/30/2011'
+);
+
+assert.sameValue(
+  instance.toLocaleString('en-US', { timeZone: 'Pacific/Apia' }),
+  instance.toLocaleString('en-US')
+);
+
+assert.sameValue(
+  instance.toLocaleString('en-US', { timeZone: 'Pacific/Apia' }),
+  instance.toLocaleString('en-US', { timeZone: 'UTC' })
+);

test/intl402/Temporal/PlainDateTime/prototype/toLocaleString/ignore-timezone.js

@@ -0,0 +1,58 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.tolocalestring
+description: Temporal.PlainDateTime should be interpreted and formatted as wall-clock time
+features: [Temporal]
+---*/
+
+// A non-existent date in the Pacific/Apia timezone.
+const instance1 = Temporal.PlainDateTime.from( { year: 2011, month: 12, day: 30 });
+
+assert.sameValue(
+  instance1.toLocaleString('en-US', { timeZone: 'Pacific/Apia' }),
+  '12/30/2011, 12:00:00 AM'
+);
+
+assert.sameValue(
+  instance1.toLocaleString('en-US', { timeZone: 'Pacific/Apia' }),
+  instance1.toLocaleString('en-US')
+);
+
+assert.sameValue(
+  instance1.toLocaleString('en-US', { timeZone: 'Pacific/Apia' }),
+  instance1.toLocaleString('en-US', { timeZone: 'UTC' })
+);
+
+// A non-existent time in the 'America/Los_Angeles' timezone.
+const instance2 = new Temporal.PlainDateTime(2026, 3, 8, 2, 30);
+
+assert.sameValue(
+  instance2.toLocaleString('en-US'),
+  instance2.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' })
+)
+
+assert.sameValue(
+  instance2.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' }),
+  '3/8/2026, 2:30:00 AM'
+)
+
+assert.sameValue(
+  instance2.toLocaleString('en-US', { timeZone: 'UTC' }),
+  instance2.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' })
+)
+
+// Creating the instance from a datestring with an offset has no effect.
+
+const instance3 = Temporal.PlainDateTime.from('2026-03-29T02:30:15+01:00');
+
+assert.sameValue(
+  instance3.toLocaleString('en', { timeStyle: 'long' }),
+  '2:30:15 AM'
+);
+
+assert.sameValue(
+  instance3.toLocaleString('en', { timeStyle: 'long' }),
+  instance3.toLocaleString('en', { timeStyle: 'long', timeZone: 'America/New_York' }),
+);

test/intl402/Temporal/PlainTime/prototype/toLocaleString/ignore-timezone.js

@@ -0,0 +1,40 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaintime.prototype.tolocalestring
+description: Temporal.PlainTime should be interpreted and formatted as wall-clock time
+features: [Temporal]
+---*/
+
+// A non-existent time in the 'America/Los_Angeles' timezone.
+const instance1 = Temporal.PlainTime.from('2026-03-29T02:30:00');
+
+assert.sameValue(
+  instance1.toLocaleString('en-US'),
+  instance1.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' })
+)
+
+assert.sameValue(
+  instance1.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' }),
+  '2:30:00 AM'
+)
+
+assert.sameValue(
+  instance1.toLocaleString('en-US', { timeZone: 'UTC' }),
+  instance1.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' })
+)
+
+// Creating the instance from a datestring with an offset has no effect.
+
+const instance2 = Temporal.PlainTime.from('2026-03-29T02:30:15+01:00');
+
+assert.sameValue(
+  instance2.toLocaleString('en', { timeStyle: 'long' }),
+  '2:30:15 AM'
+);
+
+assert.sameValue(
+  instance2.toLocaleString('en', { timeStyle: 'long' }),
+  instance2.toLocaleString('en', { timeStyle: 'long', timeZone: 'America/New_York' }),
+);