Local Mean Time offset truncates precision

[ysangkok]

It seems inconsistent that the half-minute is applied to the timestamp, but omitted from the offset:

> const zonedDateTime = Temporal.PlainDate.from({month: 1, day: 1, year: 1970}).toZonedDateTime("UTC");
> zonedDateTime.withTimeZone("Africa/Monrovia")
Temporal.ZonedDateTime {_repr_: 'Temporal.ZonedDateTime <1969-12-31T23:15:30-00:45[Africa/Monrovia]>'}

If the user supplies data that has no time of day information, I think it is reasonable for them to expect the offset + the local time to equal what they originally supplied.

The output of this calculation seems to violate that rule, since the offset omits the half minute.

[ptomato]

Hi! This is because the RFC 9557 string format doesn't allow HH:MM:SS in the offset part, so when we serialize a ZonedDateTime to a string, we don't want to output a string that other clients may not be able to parse. Temporal does accept the string 1969-12-31T23:15:30-00:44:30[Africa/Monrovia] and hopefully future improvements to RFC 9557 will allow outputting it as well.

The -00:45 string is still deserialized correctly when appropriate for the given time zone — see the note about MATCH-MINUTES in InterpretISODateTimeOffset.

More background reading on this:

• Maximum precision of offset time zones #2593
• https://github.com/tc39/proposal-temporal/blob/main/meetings/agenda-minutes-2023-06-08.md#maximum-precision-of-offset-time-zones-2593

[ysangkok]

Ok, it sounds like you're already aware of this. Should I close this then?

[justingrant]

Yep, I'll close this issue. Thanks for the feedback!