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Add coverage for remapping of eras in Temporal dates
See note 2 on eras in CalendarResolveFields: https://tc39.es/proposal-temporal/#sec-temporal-calendarresolvefields This behaviour was not yet covered by any test262 tests. Based on Anba's tests from #4080 but with different behaviour.
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‎test/intl402/Temporal/PlainDate/from/remapping-era.js‎

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// Copyright (C) 2025 Igalia, S.L. All rights reserved.
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// This code is governed by the BSD license found in the LICENSE file.
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/*---
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esid: sec-temporal.plaindate.from
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description: Test remapping behaviour of regnal eras
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info: |
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CalendarDateFromFields:
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1. Perform ? CalendarResolveFields(_calendar_, _fields_, _date_).
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2. Let result be ? CalendarDateToISO(_calendar_, _fields_, _overflow_).
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CalendarResolveFields:
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When the fields of _fields_ are inconsistent with respect to a non-unset
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_fields_.[[Era]], it is recommended that _fields_.[[Era]] and
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_fields_.[[EraYear]] be updated to resolve the inconsistency by lenient
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interpretation of out-of-bounds values (rather than throwing a *RangeError*),
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which is particularly useful for consistent interpretation of dates in
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calendars with regnal eras.
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includes: [temporalHelpers.js]
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features: [Temporal]
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---*/
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// Based on a test originally by André Bargull <[email protected]>
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// Notes:
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// - "heisei" era started January 8, 1989.
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// - "reiwa" era started May 1, 2019.
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for (const overflow of ["constrain", "reject"]) {
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function test(fields) {
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return Temporal.PlainDate.from({ ...fields, calendar: "japanese" }, { overflow });
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}
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// Reiwa era started in month 5 of the year, so the era of month 1 is remapped
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// to be the correct one for the month.
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TemporalHelpers.assertPlainDate(
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test({ era: "reiwa", eraYear: 1, monthCode: "M01", day: 24 }),
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2019, 1, "M01", 24,
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"Reiwa 1 before May is mapped to Heisei 31",
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"heisei", 31
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);
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// Reiwa era started on the first day of the month, so day 1 does not need
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// remapping.
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TemporalHelpers.assertPlainDate(
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test({ era: "reiwa", eraYear: 1, monthCode: "M05", day: 1 }),
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2019, 5, "M05", 1,
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"05-01 Reiwa 1 is not remapped",
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"reiwa", 1
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);
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// Heisei era started on the eighth day of the month. So on previous days the
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// era is remapped to be the correct one for the day.
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TemporalHelpers.assertPlainDate(
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test({ era: "heisei", eraYear: 1, monthCode: "M01", day: 4 }),
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1989, 1, "M01", 4,
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"01-04 Heisei 1 is remapped to 01-04 Showa 64",
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"showa", 64
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);
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// Era year past the end of the Heisei era is remapped to Reiwa era
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TemporalHelpers.assertPlainDate(
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test({ era: "heisei", eraYear: 37, monthCode: "M04", day: 25 }),
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2025, 4, "M04", 25,
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"Heisei 37 is remapped to Reiwa 7",
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"reiwa", 7
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);
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// Zero year in a 1-based era is remapped to the previous era
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TemporalHelpers.assertPlainDate(
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test({ era: "reiwa", eraYear: 0, monthCode: "M04", day: 25 }),
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2018, 4, "M04", 25,
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"Reiwa 0 is remapped to Heisei 30",
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"heisei", 30
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);
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// Negative year in a forward-counting era is remapped to the previous era
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TemporalHelpers.assertPlainDate(
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test({ era: "reiwa", eraYear: -20, monthCode: "M04", day: 25 }),
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1998, 4, "M04", 25,
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"Reiwa -20 is remapped to Heisei 10",
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"heisei", 10
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);
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// Test the last two things for Gregorian eras as well
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function testGregorian(fields) {
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return Temporal.PlainDate.from({ ...fields, calendar: "gregory" }, { overflow });
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}
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TemporalHelpers.assertPlainDate(
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testGregorian({ era: "ce", eraYear: 0, monthCode: "M04", day: 25 }),
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0, 4, "M04", 25,
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"0 CE is remapped to 1 BCE",
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"bce", 1
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);
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TemporalHelpers.assertPlainDate(
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testGregorian({ era: "ce", eraYear: -20, monthCode: "M04", day: 25 }),
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-20, 4, "M04", 25,
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"-20 CE is remapped to 21 BCE",
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"bce", 21
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);
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// Zero year in a backwards-counting era is remapped to the next era
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TemporalHelpers.assertPlainDate(
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testGregorian({ era: "bce", eraYear: 0, monthCode: "M04", day: 25 }),
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1, 4, "M04", 25,
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"0 BCE is remapped to 1 CE",
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"ce", 1
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);
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// Negative year in a backwards-counting era is remapped to the next era
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TemporalHelpers.assertPlainDate(
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testGregorian({ era: "bce", eraYear: -20, monthCode: "M04", day: 25 }),
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21, 4, "M04", 25,
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"-20 BCE is remapped to 21 CE",
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"ce", 21
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);
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}

‎test/intl402/Temporal/PlainYearMonth/from/remapping-era.js‎

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// Copyright (C) 2025 Igalia, S.L. All rights reserved.
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// This code is governed by the BSD license found in the LICENSE file.
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/*---
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esid: sec-temporal.plainyearmonth.from
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description: Test remapping behaviour of regnal eras
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info: |
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CalendarYearMonthFromFields:
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1. Perform ? CalendarResolveFields(_calendar_, _fields_, ~year-month~).
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2. Let _firstDayIndex_ be the 1-based index of the first day of the month
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described by _fields_ (i.e., 1 unless the month's first day is skipped by
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this calendar.)
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3. Set _fields_.[[Day]] to _firstDayIndex_.
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4. Let result be ? CalendarDateToISO(_calendar_, _fields_, _overflow_).
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CalendarResolveFields:
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When the fields of _fields_ are inconsistent with respect to a non-unset
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_fields_.[[Era]], it is recommended that _fields_.[[Era]] and
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_fields_.[[EraYear]] be updated to resolve the inconsistency by lenient
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interpretation of out-of-bounds values (rather than throwing a *RangeError*),
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which is particularly useful for consistent interpretation of dates in
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calendars with regnal eras.
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includes: [temporalHelpers.js]
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features: [Temporal]
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---*/
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// Based on a test originally by André Bargull <[email protected]>
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// Notes:
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// - "heisei" era started January 8, 1989.
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// - "reiwa" era started May 1, 2019.
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for (const overflow of ["constrain", "reject"]) {
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function test(fields) {
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return Temporal.PlainYearMonth.from({ ...fields, calendar: "japanese" }, { overflow });
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}
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// Reiwa era started in month 5 of the year, so the era of month 1 is remapped
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// to be the correct one for the month.
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TemporalHelpers.assertPlainYearMonth(
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test({ era: "reiwa", eraYear: 1, monthCode: "M01" }),
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2019, 1, "M01",
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"Reiwa 1 before May is mapped to Heisei 31",
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"heisei", 31, /* reference day = */ 1
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);
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// Reiwa era started on the first day of the month, so the reference day 1
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// does not need remapping.
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TemporalHelpers.assertPlainYearMonth(
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test({ era: "reiwa", eraYear: 1, monthCode: "M05" }),
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2019, 5, "M05",
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"reference day is 1",
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"reiwa", 1, /* reference day = */ 1
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);
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// Heisei era started on the eighth day of the month, but PlainYearMonth
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// references the first day of the month. So the era is remapped to be the
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// correct one for the reference day.
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TemporalHelpers.assertPlainYearMonth(
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test({ era: "heisei", eraYear: 1, monthCode: "M01" }),
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1989, 1, "M01",
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"01-01 Heisei 1 is remapped to 01-01 Showa 64",
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"showa", 64, /* reference day = */ 1
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);
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// Era year past the end of the Heisei era is remapped to Reiwa era
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TemporalHelpers.assertPlainYearMonth(
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test({ era: "heisei", eraYear: 37, monthCode: "M04" }),
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2025, 4, "M04",
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"Heisei 37 is remapped to Reiwa 7",
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"reiwa", 7, /* reference day = */ 1
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);
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// Zero year in a 1-based era is remapped to the previous era
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TemporalHelpers.assertPlainYearMonth(
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test({ era: "reiwa", eraYear: 0, monthCode: "M04" }),
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2018, 4, "M04",
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"Reiwa 0 is remapped to Heisei 30",
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"heisei", 30, /* reference day = */ 1
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);
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// Negative year in a forwards-counting era is remapped to the previous era
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TemporalHelpers.assertPlainYearMonth(
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test({ era: "reiwa", eraYear: -20, monthCode: "M04" }),
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1998, 4, "M04",
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"Reiwa -20 is remapped to Heisei 10",
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"heisei", 10, /* reference day = */ 1
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);
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// Test the last two things for Gregorian eras as well
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function testGregorian(fields) {
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return Temporal.PlainYearMonth.from({ ...fields, calendar: "gregory" }, { overflow });
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}
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TemporalHelpers.assertPlainYearMonth(
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testGregorian({ era: "ce", eraYear: 0, monthCode: "M04" }),
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0, 4, "M04",
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"0 CE is remapped to 1 BCE",
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"bce", 1, /* reference day = */ 1
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);
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TemporalHelpers.assertPlainYearMonth(
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testGregorian({ era: "ce", eraYear: -20, monthCode: "M04" }),
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-20, 4, "M04",
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"-20 CE is remapped to 21 BCE",
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"bce", 21, /* reference day = */ 1
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);
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// Zero year in a backwards-counting era is remapped to the next era
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TemporalHelpers.assertPlainYearMonth(
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testGregorian({ era: "bce", eraYear: 0, monthCode: "M04" }),
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1, 4, "M04",
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"0 BCE is remapped to 1 CE",
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"ce", 1, /* reference day = */ 1
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);
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// Negative year in a backwards-counting era is remapped to the next era
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TemporalHelpers.assertPlainYearMonth(
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testGregorian({ era: "bce", eraYear: -20, monthCode: "M04" }),
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21, 4, "M04",
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"-20 BCE is remapped to 21 CE",
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"ce", 21, /* reference day = */ 1
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);
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}

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