What ISO week is December 31, 2029 in?
December 31, 2029 is a Monday, so it starts ISO week 1 of 2030 — a calendar date in one year, an ISO week in the next.
Worked from a fixed input so the numbers stay put: 2029-12-31. Try your own on the home page.
2029-12-31 (Monday) is in ISO week 1 of 2030.
- Calendar date
- 2029-12-31
- ISO year
- 2030
- ISO week
- 1
- ISO weekday
- 1 (Monday)
- Weeks in that ISO year
- 52
This date's ISO week-numbering year (2030) is not its calendar year (2029)
2029-12-31 falls in ISO week 1 of 2030, not 2029, because ISO week 1 is defined by which week contains the year's first Thursday, not by January 1st. This is the single most common source of disagreement between "week number" tools.
Fix: Report the ISO year alongside the week number — "week 52 of 2025", not just "week 52".
Other examples
Not a general calendar or scheduling tool, and it does not know about business fiscal-year or retail 4-4-5 week numbering, which are different systems that also call themselves "week numbers." It has no concept of time of day or timezone — it operates on calendar dates only. Not for US-style "week of the month" or non-ISO week-start conventions (e.g. Sunday-start week numbering used by some US calendars).