Leap year and week number
Whether a year is a leap year, the ISO week number, and the day of the week for any date.
How it works
Tells you whether a year is a leap year, gives the ISO 8601 week number for a date, and names the weekday.
The leap year rule has three parts, and the third catches almost everyone: divisible by 4 is a leap year, unless divisible by 100, unless also divisible by 400. So 1900 was not a leap year and 2000 was. The correction exists because the tropical year is about 365.2422 days, not 365.25 — the quarter-day approximation drifts by three days every four centuries, and the century rule removes exactly that.
ISO week numbering is not the same as "which week of the year". Week 1 is the week containing the first Thursday, so the first days of January often belong to the last week of the previous year. A year has 52 or 53 ISO weeks, and weeks always start on Monday.
Both are why date arithmetic should always use a library or the platform date object rather than being derived by hand.
This tool runs entirely in your browser. Nothing you enter is sent to a server, logged or stored, and the page keeps working with the network disconnected.
Common questions
- Was 1900 a leap year?
- No. It is divisible by 100 but not by 400. 2000 was, because it satisfies the 400 rule.
- Why do leap years skip some centuries?
- Because the tropical year is 365.2422 days, not 365.25. Adding a day every four years overshoots by about three days per four centuries, and the century rule removes them.
- Can 1 January be in week 52?
- Yes. ISO week 1 is the week containing the first Thursday, so early January dates often belong to the previous year’s final week.
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Sources
- Calculated on this page — At One Place