Proposal-temporal: Document time zone used for Instant in Intl.DateTimeFormat

[Ayc0]

How to reproduce

I created a small runkit here with the following code:

const { Temporal, Intl } = require("proposal-temporal")

const TZ = 'Australia/Sydney';
const date = new Temporal.DateTime(2020, 4, 5, 2, 0, 0).toInstant(TZ);

console.log(new Intl.DateTimeFormat('en').format(date));

Description

And it returns 4/4/2020, 5:00:00 PM. Shouldn’t this be 4/5/2020, 2:00:00 AM instead (or at least that’s what I have from France) as the time zone is specified in the instant? Or is it normal and is meant to use my native timezone?

Versions

• proposal-temporal: 0.4.0
• node 14.14.0

[Ms2ger]

It shouldn't be 2AM - Instant doesn't remember that the Sydney time zone was involved, so it only stores the UTC date/time: 2020-04-04T15:00Z. I don't think there's then supposed to be a conversion to local time, though. (The polyfill only does that because it defers to the existing in-browser implementation, which does; the spec doesn't, AFAICT.)

[sffc]

You should convert the Temporal.DateTime to a Temporal.ZonedDateTime instead of Temporal.Instant if you want it to remember "Australia/Sydney".

[ptomato]

After talking with @sffc, the conversion to local time is correct. The time zone used for the output should be, in order of priority:

1. the timeZone option (e.g. new Intl.DateTimeFormat('en', { timeZone: 'Australia/Sydney' }).format(date))
2. the -u-tz- locale extension (e.g. new Intl.DateTimeFormat('en-u-tz-ausyd').format(date)) (it actually doesn't, ECMA-402 makes an exception and ignores this key)
3. the current time zone from the environment.
   (This is the same behaviour as Intl.DateTimeFormat has for legacy Date)

[ptomato]

I will keep this ticket open, so that we clarify this in the documentation.

Thanks for the report!

[Ayc0]

Thank you for those details 🙇

[Ayc0]

You should convert the Temporal.DateTime to a Temporal.ZonedDateTime instead of Temporal.Instant if you want it to remember "Australia/Sydney".

@sffc I used toInstant because when I was playing with the polyfill to try it out, I couldn't find toZonedDateTime in DateTime's prototype. And when I try dateTime.toZondedDateTime(timezone), it crashes (runkit):

[Ms2ger]

Yeah, ZonedDateTime hasn't landed in the polyfill yet. Soon(TM).

[Ms2ger]

Also, in the current Intl section of the spec, _dateTimeFormat_.[[TimeZone]] is only taken into account in the non-temporal case. Sounds like we should fix that.

[ryzokuken]

1034 should have addressed this, so closing, but feel free to reopen.

[ryzokuken]

Also, in the current Intl section of the spec, dateTimeFormat.[[TimeZone]] is only taken into account in the non-temporal case. Sounds like we should fix that.

Fixed for Instant.

[ptomato]

I think we still need to document what's mentioned in https://github.com/tc39/proposal-temporal/issues/1021#issuecomment-713015540, so reopening.