Normative: Disallow one day long time zone offsets

[anba]

No description provided.

[codecov]

Codecov Report

Merging #2260 (f5b51cb) into main (a3a8237) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2260   +/-   ##
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  Coverage   95.01%   95.01%           
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  Files          20       20           
  Lines       10803    10803           
  Branches     1925     1925           
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  Hits        10264    10264           
  Misses        503      503           
  Partials       36       36           

Impacted Files	Coverage Δ
polyfill/lib/ecmascript.mjs	98.29% <100.00%> (ø)

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[ljharb]

Is this a timezone offset from UTC? If so, it can only range from -12 to +14 hours. If it's the difference between any two timezones, it could be as much as 26 hours.

[anba]

It is the offset for a single time zone at a specific instant. The built-in time zone object supports up-to nsPerDay - 1 long offsets:

js> new Temporal.TimeZone("+23:59:59.999999999").getOffsetNanosecondsFor(new Temporal.Instant(0n))
86399999999999

[ljharb]

Is the design intending to support possible future timezones with larger offsets than 14 hours? If so, why wouldn't "24 hours" be a valid value?

[anba]

If so, it can only range from -12 to +14 hours.

The supported range is actually even longer because of LMT (local mean time):

• https://github.com/eggert/tz/blob/0e8f0b06ac2f65d0c54a91121da09c8662cd88e6/asia#L3518
• https://github.com/eggert/tz/blob/0e8f0b06ac2f65d0c54a91121da09c8662cd88e6/northamerica#L689

const min = new Temporal.Instant(-86_40000_00000_00000_00000n);

let minOffset = +Infinity;
let maxOffset = -Infinity;
let minTimeZone;
let maxTimeZone;

for (let id of Intl.supportedValuesOf("timeZone")) {
  console.log(id);
  let tz = new Temporal.TimeZone(id);

  let instant = min;
  for (let i = 0; i < 1000; ++i) {
    let offset = tz.getOffsetNanosecondsFor(instant);

    if (offset < minOffset) {
      minOffset = offset;
      minTimeZone = id;
    }
    if (offset > maxOffset) {
      maxOffset = offset;
      maxTimeZone = id;
    }

    let next = tz.getNextTransition(instant);
    if (next === null) {
      break;
    }
    instant = next;
  }
}
console.log("minOffset:", minOffset, minTimeZone);
console.log("maxOffset:", maxOffset, maxTimeZone);

[ljharb]

Is it up to 23 hours and 59 minutes, or a smaller value?

I'm trying to understand why 24 hours is a magic illegal number but "24 hours - 1 minute" is fine.

[anba]

Is it up to 23 hours and 59 minutes, or a smaller value?

The maximum for built-in time zone objects is "+23:59:59.999999999" (and "-23:59:59.999999999" in the other direction). This can happen only when the time zone is created from an offset string.

[ljharb]

Thanks, it all makes sense now :-)

[ptomato]

Thanks, good catch.

Would you mind splitting out the editorial change into a separate pull request so we can merge it ahead of presenting the normative change?

[ptomato]

Tests are in tc39/test262#3669.

[ptomato]

This achieved consensus in the July 2022 TC39 plenary meeting. I've added a commit that updates the reference code, and pulls in the new test262 tests.