An ordinal date is a date format from ISO 8601 that identifies a day by its day-of-year number. The range is 1 to 365 in common years and 1 to 366 in leap years.
Format
The ordinal date format is YYYY-DDD. The year is 4 digits. The day-of-year is 3 digits, zero-padded. For example, 2026-150 is the 150th day of 2026, which is 30 May 2026. The basic format without a hyphen is YYYYDDD, used in some legacy systems.
Where ordinal dates are used
Ordinal dates appear in 3 technical fields:
- Aviation — flight plans and aircraft maintenance logs.
- Meteorology — climate data series and weather station records.
- Astronomy — observation logs and ephemeris files.
These fields prefer a single counter over month and day pairs because day-of-year arithmetic is simpler for elapsed-time calculations.
Distinct from the calendar date
The ordinal date is not the same as the calendar date YYYY-MM-DD. Both refer to the same underlying day, but the ordinal form omits the month. Conversion is direct: 30 May 2026 is day 150 because January through April hold 120 days, plus 30 days in May. Software libraries handle the conversion automatically.