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January 1st is not always on the first week of the year (www.jpvalencia.com)
submitted 1 month 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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[-] csolisr@hub.azkware.net 1 points 1 month ago

Did they settle for Thursday as the "middle of the week" specifically because ISO 8601 starts the week count on Mondays?

this post was submitted on 24 Mar 2026
134 points (97.9% liked)

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