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Fix ZDT diff issue with ymd travel with opposite sign of ns travel #3317

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Fix ZDT diff issue with ymd travel with opposite sign of ns travel #3317

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28 changes: 20 additions & 8 deletions polyfill/lib/ecmascript.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -3104,16 +3104,19 @@ export function DifferenceZonedDateTime(ns1, ns2, timeZone, calendar, largestUni
// Convert start/end instants to datetimes
const isoDtStart = GetISODateTimeFor(timeZone, ns1);
const isoDtEnd = GetISODateTimeFor(timeZone, ns2);
const isoDtLexSign = CompareISODate(isoDtStart.isoDate, isoDtEnd.isoDate);

// If year-month-day values are same-day,
// there's no point involving the calendar/timezone for zdt->pdt conversions.
// It also avoids a situation where dayCorrection backs up too far on same-day diffs
// with reverse-direction wallclock delta due to DST:
// https://github.com/tc39/proposal-temporal/issues/3141
if (
isoDtStart.isoDate.year === isoDtEnd.isoDate.year &&
isoDtStart.isoDate.month === isoDtEnd.isoDate.month &&
isoDtStart.isoDate.day === isoDtEnd.isoDate.day
// If year-month-day values are same-day,
// there's no point involving the calendar/timezone for zdt->pdt conversions.
// It also avoids a situation where dayCorrection backs up too far on same-day diffs
// with reverse-direction wallclock delta due to DST:
// https://github.com/tc39/proposal-temporal/issues/3141
isoDtLexSign === 0 ||
// If year-month-day diff is opposite sign from overall nanosecond diff, then the time zone
// likely repeated a whole day (ex: America/Adak). In this case, do all math in time realm.
// https://github.com/tc39/proposal-temporal/issues/3311
isoDtLexSign !== sign
) {
Comment on lines +3107 to 3120

@ptomato ptomato May 16, 2026

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These two conditions can probably collapse into if (CompareISODate(isoDtStart.isoDate, isoDtEnd.isoDate) === -sign)?

const timeDuration = new TimeDuration(nsDiff);
return { date: ZeroDateDuration(), time: timeDuration };
Expand Down Expand Up @@ -3153,6 +3156,15 @@ export function DifferenceZonedDateTime(ns1, ns2, timeZone, calendar, largestUni
// Convert intermediate datetime to epoch-nanoseconds (may disambiguate)
const intermediateNs = GetEpochNanosecondsFor(timeZone, intermediateDateTime, 'compatible');

// For when intermediate datetime backs up too far beyond the starting point, likely due to a
// time zone's repeated day (ex: America/Adak). In this case, do all math in time realm.
const intermediateNsDiff = intermediateNs.subtract(ns1);
const intermediateSign = intermediateNsDiff.lt(0) ? 1 : -1;
if (intermediateSign !== sign) {
const timeDuration = new TimeDuration(nsDiff);
return { date: ZeroDateDuration(), time: timeDuration };
}

// Compute the nanosecond diff between the intermediate instant and the final destination
timeDuration = TimeDuration.fromEpochNsDiff(ns2, intermediateNs);

Expand Down
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