Also: January is not always the 1st month, sometimes it is the 0th.
1/1/2026 can be both Jan 1st, and Feb 1st.
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.
Even better: two weeks with the same week number can exist in the same year.
How is that? Do you mean that week 1 can occur twice? Or is there another issue?
IIUC you can have days at the start and the end of the year that are all in week 1, and those are two different weeks. But one of those would be, let's say, week 1 year 2025, and the other would be week 1 year 2026. According to the blog post if you have a week 1 at the end of a year the ISO year number for that week is the next year.
It only happens with ISO weeks. An example is 2006. Each of the weeks belong to different years (so 2005W52 and 2006W52 are different), but each can contain days from another year. So for example Sunday 2006-01-01 is part of week 52 of 2005. Week 1 starts on January 2. Then at the end of 2006 you have another week 52, but that week is actually part of 2006.
It's a bit of a cheeky thing to point out, because at no point is a day in two different weeks, and the week itself only belongs to one year. It's just that you can't assume that any given day belongs to the same year as the week it is in. That is: 2006-01-01 is in 2005W52 not 2006W01.
Fuck, I just wrote an app to help HR process data and I had to, without using Ai, define the first week of the year to match that ISO spec. Week 53 was a fun one but I now have to cover two weeks with the same number.
Yep, but the year of week will be different for each.
Use a date library. Life is too short to make your own. The new Temporal library in JavaScript is quite good.
It's written in C++ lol
fuck sake.
thanks for the heads up
Week numbers can also vary by country. Not always, but sometimes, as we're not all in agreement on what constitutes the first week of the year.
Something most people in international organisations and teams get to find out the hard way at least once.
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
Is this a thing outside the USA? It always bugs me when apps default to this. How does it make any sense for the week to start with the weekend?
A string has two ends
I've generally seen Sunday to be the start of the week here, my current headcanon for it is that Sunday and Saturday sort of book-end on both sides of the week, hence week-end_s_ plural
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