JavaScript 日期差一天？用 Intl.DateTimeFormat 分開語系與時區

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import assert from 'node:assert/strict';
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function readPlainDate(value) {
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  if (typeof value !== 'string' || value.length !== 10 || !/^[0-9]{4}-[0-9]{2}-[0-9]{2}$/.test(value)) {
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    throw new TypeError('Expected YYYY-MM-DD');
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  }
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  const [year, month, day] = value.split('-').map(Number);
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  if (year < 1 || month < 1 || month > 12 || day < 1 || day > 31) {
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    throw new RangeError('Date outside the supported range');
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  }
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  const probe = new Date(0);
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  probe.setUTCFullYear(year, month - 1, day);
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  if (probe.getUTCFullYear() !== year ||
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      probe.getUTCMonth() !== month - 1 || probe.getUTCDate() !== day) {
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    throw new RangeError('Calendar date does not exist');
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  }
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  return value;
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}
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for (const value of ['2024-02-29', '2000-02-29', '0099-01-01', '0001-01-01', '9999-12-31']) {
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  assert.equal(readPlainDate(value), value);
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}
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for (const value of ['2026-02-30', '2025-02-29', '1900-02-29', '2026-04-31', '0000-01-01', '2026-00-01']) {
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  assert.throws(() => readPlainDate(value), RangeError);
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}
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for (const value of ['2026-2-01', '2026-01-01T00:00:00Z', ' 2026-01-01', '2026-01-01\n', null, 20260101]) {
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  assert.throws(() => readPlainDate(value), TypeError);
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}
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assert.equal(new Date(Date.UTC(99, 0, 1)).getUTCFullYear(), 1999);
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console.log('純日期格式、閏年與年份邊界通過');

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const instant = new Date('2026-10-19T16:30:00Z');
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const formatter = new Intl.DateTimeFormat('zh-TW', {
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  timeZone: 'Asia/Taipei',
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  dateStyle: 'medium',
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  timeStyle: 'short',
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});
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console.log(formatter.format(instant));
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console.log(instant.toISOString()); // 仍是 2026-10-19T16:30:00.000Z

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function dateKeyAt(instant, timeZone) {
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  if (!(instant instanceof Date) || !Number.isFinite(instant.getTime())) {
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    throw new TypeError('需要有效的 Date 時間點');
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  }
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  const formatter = new Intl.DateTimeFormat('en', {
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    timeZone,
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    calendar: 'gregory',
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    numberingSystem: 'latn',
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    year: 'numeric',
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    month: '2-digit',
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    day: '2-digit',
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  });
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  const parts = Object.fromEntries(
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    formatter.formatToParts(instant)
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      .filter(({ type }) => ['year', 'month', 'day'].includes(type))
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      .map(({ type, value }) => [type, value]),
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  );
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  return `${parts.year}-${parts.month}-${parts.day}`;
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}
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const instant = new Date('2026-10-19T16:30:00Z');
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if (dateKeyAt(instant, 'Asia/Taipei') !== '2026-10-20') {
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  throw new Error('台北日期錯誤');
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}
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if (dateKeyAt(instant, 'UTC') !== '2026-10-19') {
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  throw new Error('UTC 日期錯誤');
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}

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const formatter = new Intl.DateTimeFormat('en-US', {
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  timeZone: 'America/New_York',
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  hour: '2-digit',
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  minute: '2-digit',
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  hourCycle: 'h23',
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  timeZoneName: 'shortOffset',
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});
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const a = new Date('2026-11-01T05:30:00Z');
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const b = new Date('2026-11-01T06:30:00Z');
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console.log(formatter.format(a)); // 此環境：01:30 GMT-4
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console.log(formatter.format(b)); // 此環境：01:30 GMT-5
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if (a.getTime() === b.getTime()) throw new Error('應為不同時間點');