January 1st is not always on the first week of the year (www.jpvalencia.com)

submitted 13 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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[-] spartanatreyu@programming.dev 1 points 6 hours ago

Also: January is not always the 1st month, sometimes it is the 0th.

1/1/2026 can be both Jan 1st, and Feb 1st.

this post was submitted on 24 Mar 2026

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