1962010 - Temporal's getTimeZoneTransition method returns moment without offset transition

Status: RESOLVED FIXED

(Core :: JavaScript: Standard Library, defect, P2)

Description

[[:fabon]]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0

Steps to reproduce:

for (const timeZone of Intl.supportedValuesOf("timeZone")) {
  const start = Temporal.PlainDate.from('1800-01-01').toZonedDateTime(timeZone);
  const end = Temporal.Now.plainDateISO().add({ years: 5 }).toZonedDateTime(timeZone);
  let current = start;
  while (true)  {
    const next = current.getTimeZoneTransition('next');
    if (next === null || Temporal.ZonedDateTime.compare(next, end) > 0) {
      break;
    }
    if (current.offsetNanoseconds === next.offsetNanoseconds) {
      // the returned moment doesn't correspond to a UTC offset change
      console.log(next.subtract({ seconds: 1 }), next);
    }
    current = next;
  }
}

Actual results:

hundreds of ZonedDateTime are logged in console like below:

Temporal.ZonedDateTime 1977-10-20T23:59:59+01:00[Africa/Algiers] Temporal.ZonedDateTime 1977-10-21T00:00:00+01:00[Africa/Algiers]

Expected results:

no log on console

Comment 1

[[:fabon]]

I didn't investigate all cases, but it seems that some cases correspond to the DST switch without offset transition, which getTimeZoneTransition shouldn't return (if I understand the spec correctly).

https://tc39.es/proposal-temporal/#sec-temporal-getnamedtimezonenexttransition

A transition is a point in time where the UTC offset of a time zone changes, for example when Daylight Saving Time starts or stops. The returned value t represents the first nanosecond where the new UTC offset is used in this time zone, not the last nanosecond where the previous UTC offset is used.

Comment 2

[André Bargull [:anba]]

Good question! I've filed https://github.com/tc39/proposal-temporal/issues/3105 to make this case more explicit in the Temporal spec.

Comment 3

[André Bargull [:anba]]

.

Comment 4

[[:fabon]]

Now the spec clarify what 'transition' is, and also there is a test case for this in test262.

https://github.com/tc39/proposal-temporal/pull/3108
https://github.com/tc39/test262/pull/4466

Comment 5

[André Bargull [:anba]]

Attachment: Bug 1962010: Skip over time zone transition which don't change the time zone offset. r=#spidermonkey-reviewers!

ICU4C returns transitions for all time zone rule changes, even if the effective
time zone offset didn't change. Temporal wants us to ignore these transitions.

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/8270dfc2ffcd
https://hg.mozilla.org/integration/autoland/rev/90129c1a89b3
Skip over time zone transition which don't change the time zone offset. r=spidermonkey-reviewers,dminor

Comment 7

[amarc]

https://hg.mozilla.org/mozilla-central/rev/90129c1a89b3