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resepct utc when passed to tz #2949

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resepct utc when passed to tz #2949

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resepct utc when passed to tz
Ayo1984 Oct 29, 2025
17ab465
Merge branch 'dev' into fix/timezone-utc-default-tz
iamkun Nov 4, 2025
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8 changes: 8 additions & 0 deletions src/plugin/timezone/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -140,6 +140,14 @@ export default (o, c, d) => {
// timestamp number || js Date || Day.js
return d(input).tz(timezone)
}

const hasExplicitOffset = /[Zz]|[+-]\d\d:?(\d\d)?$/.test(input)
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@iamkun iamkun Nov 20, 2025 •
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const hasTimezoneInfo = /Z|[+-]\d{2}:?\d{2}$/.test(input)
if (hasTimezoneInfo && !parseFormat) {
return d(input).tz(timezone)
}
Can we fix it in this way?

  • Detect if the input string has timezone markers (Z or +HH:MM)
  • If yes & no custom format: Let dayjs parse it normally (which correctly handles UTC), then convert to target timezone
  • If no: Keep existing behavior (treat as local time in target timezone)

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@Ayo1984 Ayo1984 Nov 27, 2025

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Yeah sounds good, will run a few tests

if (hasExplicitOffset) {
const utcDate = d.utc(input, parseFormat)
utcDate.$x.$timezone = timezone
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@Xiphoseer Xiphoseer Nov 7, 2025

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Doing it this way doesn't set the offset or any of the time fields so the resulting object likely doesn't do what you think it does here. It's also not covered by test cases, your input is UTC (not an offset), toISOString is always UTC, so comparing it to moment will pass with any value for timezone.

return utcDate
}

const localTs = d.utc(input, parseFormat).valueOf()
const [targetTs, targetOffset] = fixOffset(localTs, previousOffset, timezone)
const ins = d(targetTs).utcOffset(targetOffset)
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22 changes: 21 additions & 1 deletion test/plugin/timezone.test.js
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Original file line number Diff line number Diff line change
@@ -1,8 +1,8 @@
import MockDate from 'mockdate'
import moment from 'moment-timezone'
import dayjs from '../../src'
import timezone from '../../src/plugin/timezone'
import customParseFormat from '../../src/plugin/customParseFormat'
import timezone from '../../src/plugin/timezone'
import utc from '../../src/plugin/utc'

dayjs.extend(utc)
Expand All @@ -22,6 +22,7 @@ const VAN = 'America/Vancouver'
const DEN = 'America/Denver'
const TOKYO = 'Asia/Tokyo'
const PARIS = 'Europe/Paris'
const MELBOURNE = 'Australia/Melbourne'

describe('Guess', () => {
it('return string', () => {
Expand Down Expand Up @@ -352,3 +353,22 @@ describe('UTC timezone', () => {
expect(dayjs2.format()).toBe(moment2.format())
})
})

describe('timezone: parsing UTC input when default timezone is set', () => {
beforeEach(() => {
dayjs.tz.setDefault(MELBOURNE)
moment.tz.setDefault(MELBOURNE)
})
afterEach(() => {
dayjs.tz.setDefault()
moment.tz.setDefault()
})

it('should honor Z (UTC) when parsing with tz()', () => {
const input = '2025-10-24T05:00:00Z'
const d = dayjs.tz(input, MELBOURNE)
const m = moment.tz(input, MELBOURNE)

expect(d.toISOString()).toBe(m.toISOString())
})
})