Day of Year Calculator
Find the ordinal day number (1–366) for any date.
What is Day of Year?
The day of year (also called ordinal date or Julian day number — not to be confused with Julian Date in astronomy) is the sequential day count from January 1 (day 1) to December 31 (day 365 or 366 in leap years). It's used in agriculture, meteorology, military date-time groups, and ISO 8601 ordinal date format (YYYY-DDD).
Leap Years
In a leap year, February has 29 days, making the year 366 days total. This shifts all day numbers from March 1 onward by one compared to common years. A year is a leap year if it's divisible by 4, except centuries (divisible by 100), unless also divisible by 400.
Where Day of Year is Used
ISO 8601 ordinal
YYYY-DDD format (e.g. 2024-197)
Satellite imagery
NASA/NOAA file naming conventions
Manufacturing
Batch/lot date codes on products
Military DTG
Date-time group in operational messages