ISO 8601 calendar
ISO Week Number Calculator
Apply ISO 8601 rules, where weeks start Monday and week 1 contains the first Thursday.
✓ Vos calculs de date et d’heure ne sont pas enregistrés.
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Mode d’emploi
Find the ISO week number, week-year, start and end for any date. Inputs are validated and calculations use PHP’s calendar and IANA time-zone support.
Example
Choose realistic dates or zones, calculate, then copy or bookmark the resulting URL. The query values make a calculation easy to revisit without storing it in an account.
Frequently asked questions
Can the ISO year differ from the calendar year?
Yes. Dates close to New Year can belong to the adjacent ISO week-year.
What day starts an ISO week?
Monday.