ZonedDateTime until / since assertion failure: mixed-sign values not allowed as duration fields

[phensley]

Code to reproduce:

const zone = 'America/Vancouver';

// Sunday, November 2, 2025 8:01:00 AM UTC
// 2025-11-02 01:01:00-07:00 Local
const epoch1 = 1762070460000;

// Sunday, November 2, 2025 9:00:00 AM UTC
// 2025-11-02 01:00:00-08:00 Local
const epoch2 = 1762074000000;

const date1 = Temporal.Instant.fromEpochMilliseconds(epoch2).toZonedDateTimeISO(zone);
const date2 = Temporal.Instant.fromEpochMilliseconds(epoch1).toZonedDateTimeISO(zone);

// date2.until(date1) also fails
date1.until(date2, { largestUnit: 'year', smallestUnit: 'millisecond' });

Assertion failure:

RangeError: mixed-sign values not allowed as duration fields

 ❯ ES.RejectDuration ../../node_modules/.pnpm/@js-temporal+polyfill@0.5.1/node_modules/@js-temporal/polyfill/lib/ecmascript.ts:3250:50
 ❯ new Duration ../../node_modules/.pnpm/@js-temporal+polyfill@0.5.1/node_modules/@js-temporal/polyfill/lib/duration.ts:54:5
 ❯ TemporalDurationFromInternal ../../node_modules/.pnpm/@js-temporal+polyfill@0.5.1/node_modules/@js-temporal/polyfill/lib/ecmascript.ts:3400:10
 ❯ ES.DifferenceTemporalZonedDateTime ../../node_modules/.pnpm/@js-temporal+polyfill@0.5.1/node_modules/@js-temporal/polyfill/lib/ecmascript.ts:4383:14
 ❯ Temporal.ZonedDateTime.until ../../node_modules/.pnpm/@js-temporal+polyfill@0.5.1/node_modules/@js-temporal/polyfill/lib/zoneddatetime.ts:226:12

Version info:

Node.js: 22.13.1
@js-temporal/polyfill: 0.5.1
jsbi: 4.3.2
typescript: 5.8.3

[fabon-f]

Firefox and other polyfill do the same, so I guess this behavior is spec-compliant, but I don't know this is intended by the spec or an overlooked spec bug. Reporting to the proposal repository (https://github.com/tc39/proposal-temporal) would be better.

[ptomato]

It's not necessarily the case that the behaviour is spec compliant even if Firefox and the polyfill do the same thing. We'd have to step through the spec text directly to verify that. But I think it makes sense to report it as a potential spec issue as you suggested, thanks for doing so!