Week & Day of Year
For any date, computes the ISO 8601 week number, day of year (1-366) and the quarter.
- ISO Week
- ISO Year
- Day of year
- Quarter
- Weekday
How does numbering work?
The ISO 8601 week number begins on the first week of the year that holds at least 4 days. That week may be week 1 of the current year or week 52/53 of the previous one. That's why the "ISO year" sometimes won't match the calendar year.
The day of year, meanwhile, runs from 1 to 365/366. And the quarter follows the split Q1 (Jan-Mar), Q2 (Apr-Jun), Q3 (Jul-Sep), Q4 (Oct-Dec).
Nothing leaves your browser; the math runs locally.
Week number and day of the year
In companies and in software, it's common to point to a date by its week number or day of the year instead of the usual calendar. This tool works out those numbers for any date: the ISO 8601 week, the day of the year and the quarter.
The ISO week follows its own rules (it starts on Monday, and the first week is the one holding the first Thursday), so doing the math in your head is asking for trouble. Here it's all sorted automatically, which helps with planning, reports, corporate calendars and any system that lays the year out by weeks.
When "week 32" or "day 215" gets the point across better than the calendar date, this is your precise reference. The calculation runs in the browser.
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