[jks]

Another fun fact: there are three commonly used ways to define week numbers (see `man strftime`, %U, %V, %W; %V is the one used in at least the Nordics). In some years they coincide so you might not notice that you picked the wrong one until next January.

Yet another fun fact: with %V week numbers, the date 2024-12-30 (December 30, 2024) was 2025-W01-1 (the Monday of week 1, 2025). Thus strftime needs two different ways to specify the year: %G denotes the year that goes with %V week numbers, %Y denotes the year that people usually think of when they ask "what year is it". Unfortunately %G comes before %Y on the strftime man page, so people who scan the page quickly can easily pick %G when they really want %Y. I've seen a few bugs caused by this.

I have also seen the corresponding bug in SQL, using IYYY instead of YYYY. This boggles the mind, but apparently when some people read "ISO 8601 week-numbering year", they only see "ISO 8601 ... year", think "yes, that's the date standard we use" and don't care about the "week-numbering" word in the middle.