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

[Renegade334]

Minimal repro:

new Intl.DateTimeFormat(undefined, { month: 'long', calendar: 'iso8601' }).format();

ICU 78.1: ''
ICU 77.1: 'April'