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Find the ISO 8601 week number for any date — the same numbering used in project planning, payroll, and manufacturing date codes.
The ISO 8601 standard defines week 1 of a year as the week containing that year's first Thursday — equivalently, the week containing January 4th. Weeks always run Monday to Sunday. This means the first few days of January can sometimes belong to week 52 or 53 of the previous year, and the last few days of December can belong to week 1 of the next year — which is exactly the kind of edge case that trips up a hand-rolled implementation.
Need the weekday instead of the week number? Try the Day of the Week Finder.
Related
Find the weekday and day-of-year for any date.
See the difference between two dates in weeks.
Check whether a year has 52 or 53 ISO weeks.