Incorrect UTC offset from Temporal.Now.zonedDateTimeISO in QuickJS environment and FormatJS polyfills

[mcella]

When calling Temporal.Now.zonedDateTimeISO() in a QuickJS environment, the resulting ZonedDateTime object has an incorrect UTC offset (e.g., -16:00) even when the underlying Intl polyfill is correctly configured and provides the correct offset when queried directly.

Example result for Temporal.Now in America/New_York:

2025-07-09T21:40:10.872-16:00[America/New_York]

The following script demonstrates the issue for me:

import '@formatjs/intl-getcanonicallocales/polyfill-force.js'
import '@formatjs/intl-locale/polyfill-force.js'
import '@formatjs/intl-pluralrules/polyfill-force.js'
import '@formatjs/intl-numberformat/polyfill-force.js'
import '@formatjs/intl-numberformat/locale-data/en.js'
import '@formatjs/intl-datetimeformat/polyfill-force.js'
import '@formatjs/intl-datetimeformat/locale-data/en'
import '@formatjs/intl-datetimeformat/add-all-tz.js'

import { Temporal } from 'temporal-polyfill';
globalThis.Temporal = Temporal;

const formatter = new Intl.DateTimeFormat('en-US', {
    timeZone: 'America/New_York',
    timeZoneName: 'longOffset',
});
const offsetFromIntl = formatter.formatToParts(new Date()).find(part => part.type === 'timeZoneName')?.value;
console.log('Offset from Intl.DateTimeFormat directly:', offsetFromIntl);

const nowInNewYork = Temporal.Now.zonedDateTimeISO("America/New_York");
console.log('Result from Temporal.Now.zonedDateTimeISO:', nowInNewYork.toString());

To run this script with QuickJS interpreter we need to bundle it, for example using esbuild:

npx esbuild test.js --bundle --outfile=test.bundle.js --format=iife --platform=neutral --banner:js="globalThis.Intl = {};"

And the run it using qjs:

qjs test.bundle.js

This is the output for me:

Offset from Intl.DateTimeFormat directly: GMT-4
Result from Temporal.Now.zonedDateTimeISO: 2025-07-09T21:40:10.872-16:00[America/New_York]

Note that running the same code using the js-temporal/temporal-polyfill returns the right offset on the same environment:

2025-07-10T08:17:21.062841028-04:00[America/New_York]

I dunno if the problem lies in fullcalendar/temporal-polyfill or the FormatJS polyfills, let me known if I can help in any way to debug the issue.

[arshaw]

Hi @mcella , if you run this in a more traditional browser environment, or Node, do you get the same issue?

[mcella]

Hi @arshaw, first of all thanks for your attention.

I'm getting the same issue when using Node:

$ node test.bundle.js
Offset from Intl.DateTimeFormat directly: GMT-4
Result from Temporal.Now.zonedDateTimeISO: 2025-07-29T00:27:14.422-16:00[America/New_York]

[fabon-f]

The polyfill internally uses Intl.DateTimeFormat with en-GB locale, so adding import "@formatjs/intl-datetimeformat/locale-data/en-GB.js"; will fix the issue. (I think default en locale refers to en-US which uses 12-hour clock)

[arshaw]

Good find @fabon-f. It might be worth removing reliance on the en-GB locale and instead just using using en with 24-hour clock explicitly specified in the settings.

[mcella]

Thanks @fabon-f, I can confirm that adding the en-GB import fixes the issue:

Offset from Intl.DateTimeFormat directly: GMT-4
Result from Temporal.Now.zonedDateTimeISO: 2025-09-01T05:59:49.772-04:00[America/New_York]