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January 1st is not always on the first week of the year (www.jpvalencia.com)
submitted 10 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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[-] Pissmidget@lemmy.world 19 points 10 hours ago

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.

this post was submitted on 24 Mar 2026
72 points (100.0% liked)

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