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.