The other day at work I stumbled upon this bug and thought it was worth to write a blog post about. Spoiler: It has nothing to do with timezones!
TLDR: According to ISO standard 8601 (which is what Python’s date.isocalendar().week uses for example), the first week of the year is the week with the first Thursday of the year. So sometimes the first few days of January belong to the last week of previous year, and sometimes the last few days of December belong to the first week of next year :D
Obligatory links:
Falsehoods that programmers think about dates and times: https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time
Tom Scott descends into time and timezone madness: https://www.youtube.com/watch?v=-5wpm-gesOY
I’ve personally never worked with ISO 8601 directly, and quite frankly prefer how RFC 3339 simply doesn’t deal with weeks at all.
That video is a classic. It’s exactly why I put “it has nothing to do with timezones” as subtitle for the post :) That was my first guess when I encountered the bug, before I realized it was about the ISO weeks.
And in the end of the post I also added a list of falsehoods, different from your link tho: https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b923ca