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January 1st is not always on the first week of the year (www.jpvalencia.com)
submitted 11 hours ago by null4141@programming.dev to c/programming@programming.dev
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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

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[-] bleistift2@sopuli.xyz 13 points 11 hours ago

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).

[-] ttayh@lemmy.zip 15 points 11 hours ago

The 1st day of the feek is not universal. I grew up with the week starting on sunday

[-] bleistift2@sopuli.xyz 12 points 11 hours ago

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.

[-] TehPers@beehaw.org 3 points 11 hours ago

Like ISO 8601. /s

[-] sergiu@lemmy.world 7 points 10 hours ago

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?

[-] brisk@aussie.zone 4 points 8 hours ago

A string has two ends

[-] sergiu@lemmy.world 2 points 1 hour ago

A string is reversible, a week is not. A week has a beginning and an end.

[-] technohacker@programming.dev 2 points 8 hours ago

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

[-] sergiu@lemmy.world 1 points 1 hour ago* (last edited 1 hour ago)

I'm just curious how this works in practice.

When you say something like "Let's meet up this weekend", is it implied that you will meet on Saturday, since Sunday is the other end of next week, so technically a different weekend?

[-] technohacker@programming.dev 1 points 1 hour ago

Not quite sure actually, I think we end up specifying a day or implicitly consider both of them

this post was submitted on 24 Mar 2026
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