Java's SimpleDateFormat: YYYY vs. Yyyy

Why does YYYY exist? Is there some common need I'm not familiar with?

Also, I clicked on the title because I wanted to know what format Yyyy was... I feel mislead.

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.

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.

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

> Why does YYYY exist?

In some countries (EU?) things get counted towards tax years based on it. It's probably useful for accounting software.

In case no one else noticed, Yyyy is not real. Its caused by automatic capitalization of yyyy in the HN title.

The title case formatting in the headline is making this more confusing than it should be (Yyyy should be yyyy).

A lot of analytical and financial reporting has to tie activity to its ISO week, often to roll that activity up to the correct month and year. YYYY is perfect for that.

(2019)