Developer tools
Week & Day Numbers
Find the ISO week number, ISO week-year, day of the year, days remaining and quarter for any date.
How it works
ISO 8601 weeks start on Monday. Week 1 is the week containing the year's first Thursday (equivalently, the week with 4 January). A year has 52 or 53 ISO weeks.
Because weeks don't split at New Year, the first days of January can belong to the last week of the previous ISO year, and the last days of December to week 1 of the next. Time shows the ISO week-year alongside the calendar year so this is never confusing.
Worked example
7 October 2026: ISO week 41 of 2026, day 280 of 365, quarter Q4. The week runs 5 Oct 2026 – 11 Oct 2026.
1 January 2027 is in week 53 of ISO year 2026; 30 December 2024 is in week 1 of ISO year 2025. The ISO week-year can differ from the calendar year for a few days around New Year.
Edge cases handled
- Week 53 years (2026, 2020, 2015).
- Dates whose ISO week-year differs from the calendar year.
- Leap years (day 366).
- Quarter boundaries.