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⌚ Temporal and TG2

Philip Chimento Igalia, in partnership with Bloomberg
TC39-TG2 February 2022

Refresher: Temporal

  • Stage 3 TC39 proposal
  • Introduces 7 data types for representing dates and times
  • As well, Temporal.Duration for representing lengths of time
  • Temporal.TimeZone and Temporal.Calendar allow calculations with time zones and calendars

Refresher: Temporal (2)

  • Temporal.Instant: exact time, no time zone or calendar
  • Temporal.ZonedDateTime: exact time, with time zone and calendar
  • Temporal.PlainDateTime: wall-clock time, with calendar
  • Temporal.PlainTime, Temporal.PlainDate, Temporal.PlainYearMonth, Temporal.PlainMonthDay: wall-clock time / wall-calendar date, with less information

How Temporal interacts with TG2

  • An annex to Temporal lists all modifications that would be made to Ecma-402
  • Some open questions that require a decision from TG2

Ecma-402 annex to Temporal proposal

Time Zone Names

Disregard this section; it's going to be removed.

(Originally the idea was to take IsValidTimeZoneName, CanonicalizeTimeZoneName, and DefaultTimeZone into Ecma-262.)

GetOptionsObject

This abstract operation is proposed to be moved into Ecma-262, so there's no need to keep a separate copy in Ecma-402.

DateTimeFormat

Temporal requires having a different "pattern" for each Temporal type, instead of one pattern for formatting Date, so we adapt Intl.DateTimeFormat accordingly.

(There is an issue open about storage requirements for Intl.DateTimeFormat: #602)

Calendar support

  • Without Ecma-402, Temporal only knows the ISO 8601 calendar, specified exactly
  • Add era() and eraYear() methods to Temporal.Calendar
  • Add era and eraYear properties to Plain types
  • Supersede the ISO calendar operations defined in Ecma-262

Calendar support (2)

  • Non-ISO calendar operations are specified loosely
  • Just how loosely is still a matter of debate
  • Pull request: tc39/proposal-temporal#1928

Open questions

These may or may not require discussion / action from TG2.

How to handle out-of-bounds era/eraYear

  • TG2 issue: #540
  • Rough agreement in GitHub issue

Formatting ZonedDateTime's time zone

  • Temporal issue: #2013
  • FormatDateTimePattern will have to gain a time zone argument, so that the ZonedDateTime's time zone overrides the time zone in the [[TimeZone]] internal slot of the Intl.DateTimeFormat object

Property-bag zones & calendars in 402

  • Temporal issue: #2005
  • The [[TimeZone]] and [[Calendar]] internal slots of DateTimeFormat should be Temporal instances, not strings
  • Temporal also accepts property-bag time zones and calendars that implement the protocol
const timeZone = {
    getOffsetNanosecondsFor(instant) { ... },
    getPossibleInstantsFor(dateTime) { ... },
    toString() { return "Etc/My_Zone"; }
};
const dateTime = Temporal.Now.zonedDateTimeISO(timeZone);
dateTime.toLocaleString();  // Should this code succeed or fail? What should be the result?

Calendar-specific questions

These will likely need user research from TG2.

  • What's the behaviour of date arithmetic around epagomenal days? p-t#1994
  • What should be the anchor year for the Ethiopic calendar? #534
  • What era should be used for the Hebrew calendar? #535
  • Designing unique identifiers for eras #541

Thanks!