Intl.DateTimeFormat('de-CH', {year, month}) outputs different separators between Node 24.13.0 and 24.13.1

Why this is a good first issue

Behavior change in Intl.DateTimeFormat output between Node versions due to ICU updates.

AI Summary

The issue involves inconsistent date formatting output for the de-CH locale between Node 24.13.0 and 24.13.1, specifically for partial date formats (year + month). The change is due to updates in ICU locale data, which Node.js cannot synchronize with web browsers. This affects frameworks relying on consistent date formatting for SSR/client hydration.

Issue Description

Version

24.13.1

Platform

Darwin N22134 25.3.0 Darwin Kernel Version 25.3.0: Wed Jan 28 20:48:41 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T6041 arm64

Subsystem

No response

What steps will reproduce the bug?

Intl.DateTimeFormat produces inconsistent formatting for partial date options (year + month) between Node 24.13.0 and 24.13.1. • Node 24.13.0 outputs '12.2021' (dot separator) as expected for de-CH. • Node 24.13.1 outputs '12/2021' (slash separator) for the same code. • Adding day: '2-digit' produces consistent '07.12.2021' in both versions.

This behavior change breaks SSR/client hydration in frameworks like Next.js when formatting dates for de-CH locale.

Reproduction:

// Test partial month/year formatting
console.log(
  new Intl.DateTimeFormat('de-CH', { year: 'numeric', month: '2-digit' }).format(new Date("2021-12-07"))
);

// Test full date formatting
console.log(
  new Intl.DateTimeFormat('de-CH', { year: 'numeric', month: '2-digit', day: '2-digit' }).format(new Date("2021-12-07"))
);

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

Always

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

Both Node versions should produce '12.2021' for { year, month } in de-CH locale. Separator should follow locale conventions consistently, even for partial formats.

What do you see instead?

Node 24.13.1 outputs '12/2021'

Additional information