Weekday, day of year, and ISO week date
Day of year counts January 1 as day 1. The ISO week date uses Monday as day 1, and its week-year can differ from the calendar year around New Year. Leap years contain 366 days rather than 365.
To count elapsed time between two dates, use the Date Difference Calculator .
The formula
The date is parsed at UTC midnight and checked by an exact round trip, so invalid dates are rejected rather than normalized. ISO week-year is found from the week’s Thursday, then the week number is counted from the Monday of week 1. No device time-zone adjustment is applied to the entered civil date.
Worked example
September 30, 2026 is Wednesday, day 273 of a common year, with ISO week date 2026-W40-3. January 1, 2021 is Friday but belongs to ISO week 2020-W53-5.
How to use this calculator
- Enter a complete date between years 1000 and 9999.
- Calculate to find the weekday and ordinal day of the year.
- Use the ISO result when a schedule identifies dates by week-year, week, and weekday.
The calendar matters for historical dates
Many countries adopted the Gregorian calendar at different times. This tool applies Gregorian rules to the entire accepted range, sometimes called the proleptic Gregorian calendar. A date written in a historical Julian calendar is therefore not directly interchangeable with the same numbers entered here. Establish the source calendar before researching an event’s weekday.
Leap years and century boundaries
Gregorian years divisible by 4 are leap years except centuries that are not divisible by 400. Thus 2000 was a leap year, while 1900 was not. February 29 is valid only in a leap year. The input validation checks the actual day, so an impossible date does not silently become a date in March.
Understanding the ISO week-year
ISO weeks start on Monday. Week 1 is the week containing January 4, equivalently the first Thursday of the calendar year. A late-December date can belong to next year’s week 1, and an early-January date can belong to the previous week-year. This convention is useful for weekly reporting but differs from Sunday-first calendars and local business-week rules.
Assumptions & limitations
What this calculation assumes
- Gregorian rules apply throughout the accepted year range; entered dates have no time of day.
What to keep in mind
- No Julian-calendar conversion, holiday lookup, time-zone conversion, or local week-number convention.
Common questions
Does my time zone change the result?
No. You enter a civil date, not a timestamp. UTC is used internally to avoid daylight-saving and local-time shifts.
Why can the ISO year differ from the entered year?
A week is assigned to the year containing its Thursday. Weeks that cross New Year may therefore use a different week-year.