ISO 8601 week number for any date.
This calculator becomes interactive when JavaScript is available. The page still provides its method, limits and related tools without JavaScript.
Uses the ISO 8601 rule: weeks run Monday–Sunday, and week 1 of a year is whichever week contains that year's first Thursday. That's why the first days of January sometimes belong to the previous year's final week, and the last days of December can belong to week 1 of the next year.
Standard-based. The result follows a published technical standard or the browser’s IANA time-zone data. Browser time-zone data can vary by version, so important historical or future rules should be confirmed.
Depends on the browser’s standards and time-zone database version. Repeated or nonexistent local times require an explicit choice.
Because ISO week 1 is defined by the first Thursday of the year, not by January 1 itself — if January 1 falls on Friday, Saturday, or Sunday, it's still part of the previous year's last week.
