Missing month in `Intl.DateTimeFormat` output when using `calendar: 'iso8601'` in Node v24.13.0 and later

[jasongerbes-vista]

Version

v24.15.0, v25.9.0

Platform

Darwin Jasons-MacBook-Pro-2.local 25.4.0 Darwin Kernel Version 25.4.0: Thu Mar 19 19:30:44 PDT 2026; root:xnu-12377.101.15~1/RELEASE_ARM64_T6000 arm64

Subsystem

No response

What steps will reproduce the bug?

Run the following code using Node.js 24.13.0 or later:

const dtfWithIsoCalendar = new Intl.DateTimeFormat('en-US', {
 dateStyle: 'full',
 timeStyle: 'long',
 timeZone: 'UTC',
 calendar: 'iso8601'
});

const date = new Date('2024-09-09T08:00:00Z');

console.log(dtfWithIsoCalendar.format(date));

How often does it reproduce? Is there a required condition?

The issue can be consistently reproduced under the following conditions:

• Node.js version 24.13.0 or later
• Using Intl.DateTimeFormat with calendar: 'iso8601'

The issue does not reproduce:

• On Node.js version 24.12.0 and earlier
• When using the default calendar ('gregory') instead of 'iso8601'

What is the expected behavior? Why is that the expected behavior?

Expected output:

2024 September 9, Monday at 08:00:00 AM UTC

The format() output should include the month "September".

What do you see instead?

Actual output (month is missing):

2024 9, Monday at 08:00:00 AM UTC

Additional information

No response

[fajarmf10]

I can reproduce this on Windows with Node v25.9.0 / ICU 78.2.

The smaller repro also returns an empty string when passing a date explicitly:

new Intl.DateTimeFormat(undefined, {
 month: 'long',
 calendar: 'iso8601'
}).format(new Date('2026-04-30T00:00:00Z'))

Observed output:

''

[jamiter]

Also an issue with node 26. It surfaced when using the new Temporal API:

Temporal.Now.plainDateISO().toPlainYearMonth().toLocaleString(undefined, { calendar: 'iso8601', month: 'long' })

For PlainYearMonth the default calendar is iso8601 and I have to pass it to toLocaleString, because else it's a mismatch.

Current workaround:

export function formatYearMonth(
 locale: string,
 yearMonth: Temporal.PlainYearMonth,
 options?: Intl.DateTimeFormatOptions,
): string {
 return Temporal.PlainYearMonth.from({
 calendar: 'gregory', // The default iso8601 calendar is returning an empty string for the long month name
 year: yearMonth.year,
 month: yearMonth.month,
 }).toLocaleString(locale, { ...options, calendar: 'gregory' });
}

[Renegade334]

There is discussion within ECMA-402 as to how to handle the iso8601 calendar within DateTimeFormat (tc39/ecma402#1036) as the consensus is that it should be entirely locale-independent and only support the YYYY-MM-dd date format, meaning no month or era names.

I think we can mark this as "undefined behaviour" pending clarification from the standards groups. Users are free to use JS-side workarounds if they find themselves directly impacted.

[github-actions]

This issue has been marked as stale due to 90 days of inactivity.
It will be automatically closed in 30 days if no further activity occurs. If this is still relevant, please leave a comment or update it to keep it open.