[masklinn]

The article is truly worthless for not explaining it.

YYYY is for ISO week-years e.g. December 30, 2024 was also W01 2025 in the ISO calendar. Similarly January 1st 2027 will be W53 2026.

ISO weeks are in common use in Europe.

[skissane]

This API would have been so much less error-prone if it had used symbolic names instead - e.g. “{year}”, “{weekYear}”, maybe even something like “{year:2}” for a 2 digit year.

[masklinn]

Good or bad, it’s the Unicode locale formatting langage (LDML).