Duration.p.round doesn't balance from hours (or smaller) to weeks (or larger) units

[lionel-rowe]

Duration#round appears to give the wrong answer for certain durations and options. It's also not idempotent — rounding a second time with the same options yields a different result (the originally expected one).

Example:

const now  = Temporal.ZonedDateTime.from('2023-02-21T00:00:00+00:00[UTC]')
const then = Temporal.ZonedDateTime.from('2025-06-15T00:00:00+00:00[UTC]')

const duration = now.until(then)
// PT20280H
const rounded1 = duration.round({ relativeTo: now, largestUnit: 'year' })
// P845D
const rounded2 = rounded1.round({ relativeTo: now, largestUnit: 'year' })
// P2Y3M25D

The unexpected behavior affects both:

• The experimental implementation at https://tc39.es/proposal-temporal/docs/duration.html
• The polyfill at https://github.com/js-temporal/temporal-polyfill

Surely calling round twice with the same options should yield the same result as calling it once? "845 days" is a very unexpected result given largestUnit is year.

[justingrant]

Thanks @lionel-rowe for catching this. Your second result is the correct result. Your first result is unexpected.

We'll get this fixed, although if this is a spec bug we'll need to wait to fix it in the polyfills until the spec change is OK-ed at the next TC39 committee meeting in late March.

@ptomato If I'm reading the spec correctly, this looks like a bug in the spec that the polyfills are faithfully reproducing. In Duration.p.round, hours=>days balancing happens in the call to BalanceDuration, but there's no call after that that could turn days into weeks/months/years. So if there's more than a week (or month, or year) of hours (or smaller units), then the result ends up with a large number of unbalanced days instead of balancing those days up to larger units.

Here's a simpler repro:

Temporal.Duration.from('P100D').round({ relativeTo: '2023-02-21', largestUnit: 'month' })
// P3M11D - correct result
Temporal.Duration.from('PT2400H').round({ relativeTo: '2023-02-21', largestUnit: 'month' })
// P100D - expected P3M11D