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


The first month is inputted with a 1 and exported as a 0.
The first day is inputted with a 1 and exported as a 1.
I understand that. Because the months are zero indexed. But it’s not just the first month that’s “0-indexed”. The list of months is zero indexed. Perhaps I’m arguing semantics - but was simply adding clarity for the reader.
1st becomes 0, 2nd becomes 1, 3rd becomes 2
Yeah, but when days are 1-indexed and month are 0-indexed you just feel it’s a very JavaScript thing to do
Yeah absolutely. I’m not disagreeing or saying it’s good.
I’m just correcting the strange wording.