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
If you think looking at Thursday seems like a random choice: It’s basically a majority vote among the days of the week. If Thursday is in the new year, then 4 days (Thu–Sun) are in the new year. If Thursday is in the old year, then 4 days are in the old year (Mon–Thu).
The 1st day of the feek is not universal. I grew up with the week starting on sunday
Yes, another thing in the neverending list of shit you have to learn at some point if you’re any kind of programmer.
Some people start their week on the wrong day.
Like ISO 8601. /s
When we implemented week numbers we actually had to support both systems because customers expected the number to match that in their other applications - some of which followed the standard, and some didn’t.
Also: January is not always the 1st month, sometimes it is the 0th.
1/1/2026 can be both Jan 1st, and Feb 1st.
For the downvoters, try it in your browser’s terminal:
let test = new Date("1-1-2026"); console.log(`Year: ${test.getFullYear()}, Month: ${test.getMonth()}, Day: ${test.getDate()}`); // Prints -> Year: 2026, Month: 0, Day: 1 // --- --- --- --- --- --- --- --- // test.getFullYear() returns the year, but test.getYear() only returns the number of years since 1900 // test.getMonth() returns the month, but the first month is 0-indexed // test.getDate() returns the day, 1-indexed, but test.getDay() returns the current day right now and not the day of the date objectbut the first month is 0-indexed
U mean “the months are zero indexed” and “the first month is 0”
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.
So nobody talks about leap seconds? 😅
