How to use Day of Year Calculator
- Choose date-to-ordinal or ordinal-to-date conversion.
- Enter the date, or the year and one-based ordinal day.
- Review the converted value and whether the year has 365 or 366 days.
Example: Day of Year Calculator
Calendar date 2026-10-03.
You add
Calendar date 2026-10-03.
You get
It is day 276 of the 365-day year 2026.
Supported inputs and limits
Proleptic Gregorian years 0001 through 9999. January 1 is day 1. Day 366 is valid only in a leap year. This is date-only arithmetic with no time-zone conversion, business-day counting or historical calendar-switch modeling.
Where your input is processed
This tool processes your input in this browser. Your text and files are not uploaded to UseFreeTools. Check this tool's limits for anything it may save on your device.
Ordinal days keep the year attached
A day number alone does not identify a date. The year determines leap-day placement, so ordinal-to-date conversion always requires both values.
Questions about Day of Year Calculator
Is January 1 day zero or day one?
Day one. December 31 is day 365 in a common year and day 366 in a leap year.
Why is 1900 not a leap year but 2000 is?
Gregorian century years are leap years only when divisible by 400. Other years divisible by four are leap years.
Will my browser’s time zone change the answer?
No. The calculation uses the calendar date you enter, without interpreting it as a local clock time.