Different duration balancing PlainDate::until vs Duration::round

[arshaw]

Rounding-up and balancing-up works differently depending whether PlainDate::until is used or Calendar::round is used with relativeTo. This seems very similar to this recently-closed issue.

const past = Temporal.PlainDate.from('2022-01-01')
const future = Temporal.PlainDate.from('2023-12-25')

const durationViaUntil = past.until(future, {
  largestUnit: 'year',
  smallestUnit: 'month',
  roundingMode: 'ceil'
})
console.log(durationViaUntil.toString()) // P1Y12M (undesirable result IMO)

const durationViaRound = past.until(future).round({
  relativeTo: past,
  largestUnit: 'year',
  smallestUnit: 'month',
  roundingMode: 'ceil'
})
console.log(durationViaRound.toString()) // P2Y (expected result)

[ptomato]

Thanks for reporting this. This looks like it's a bug, indeed. We need to verify whether it's actually a bug in the spec (in which case it'd need to be presented for consensus in TC39) or just a mistake in the polyfill.

[ptomato]

This is unfortunately a bug in the spec text and PlainDate.until is not the only place it occurs. It's caused by RoundDuration not being followed by BalanceDuration and/or BalanceDurationRelative as it is in Temporal.Duration.prototype.round(). Here are all the situations where it occurs, as far as I can tell:

const roundingMode = 'ceil';

// PlainDate.until and since (original example)
const pdPast = Temporal.PlainDate.from('2022-01-01');
const pdFuture = Temporal.PlainDate.from('2023-12-25');
pdPast.until(pdFuture, {largestUnit: 'year', smallestUnit: 'month', roundingMode})
  // => P1Y12M
pdPast.until(pdFuture).round({relativeTo: pdPast, largestUnit: 'year', smallestUnit: 'month', roundingMode})
  // => P2Y

// PlainDateTime.until and since
// NB. The problem exists for date units, but time units work correctly
const pdtPast = Temporal.PlainDateTime.from('2022-01-01T00');
const pdtFuture = Temporal.PlainDateTime.from('2023-12-25T00');
pdtPast.until(pdtFuture, {largestUnit: 'year', smallestUnit: 'month', roundingMode})
  // => P1Y12M
pdtPast.until(pdtFuture).round({relativeTo: pdPast, largestUnit: 'year', smallestUnit: 'month', roundingMode})
  // => P2Y

// ZonedDateTime.until and since
const zdtPast = Temporal.ZonedDateTime.from('2022-01-01T00[UTC]');
const zdtFuture = Temporal.ZonedDateTime.from('2023-12-25T00[UTC]');
zdtPast.until(zdtFuture, {largestUnit: 'year', smallestUnit: 'month', roundingMode})
  // => P1Y12M
zdtPast.until(zdtFuture).round({relativeTo: zdtPast, largestUnit: 'year', smallestUnit: 'month', roundingMode})
  // => P2Y

// PlainYearMonth.until and since
const pymPast = Temporal.PlainYearMonth.from('2022-01');
const pymFuture = Temporal.PlainYearMonth.from('2023-12');
pymPast.until(pymFuture, {largestUnit: 'year', smallestUnit: 'month', roundingIncrement: 3, roundingMode})
  // => P1Y12M
pymPast.until(pymFuture).round({relativeTo: pdPast, largestUnit: 'year', smallestUnit: 'month', roundingIncrement: 3, roundingMode})
  // => P2Y

// Duration.toString
const duration = Temporal.Duration.from('PT1H59M59.9S');
duration.toString({smallestUnit: 'second', roundingMode: 'ceil'})
  // => 'PT1H59M60S'
duration.round({smallestUnit: 'second', roundingMode}).toString()
  // => 'PT2H'

With time units, it's generally already implemented correctly (except for Duration.toString):

Temporal.PlainTime.from('00:00').until('01:59:59', {largestUnit: 'hour', smallestUnit: 'minute', roundingMode})
  // => PT2H
Temporal.Instant.fromEpochSeconds(0).until(Temporal.Instant.fromEpochSeconds(7199), {largestUnit: 'hour', smallestUnit: 'minute', roundingMode})
  // => PT2H

[justingrant]

Meeting 2023-08-17: We'll fix this and the fix has already been approved. Will be fixed in #2571 which is stacked on other PRs at the moment.