Julian Date & Day-of-Year Converter
Convert a calendar date to the astronomical Julian Day Number, Modified Julian Date and the ordinal 'Julian date' (YYDDD / day of year) used in manufacturing and the military.
How to use
- 1Enter a date to convert.
- 2Or enter a YYDDD / YYYYDDD code to get the calendar date back.
How it's calculated
Julian Day Number = days since noon UTC on 1 January 4713 BC (proleptic Julian calendar); MJD = JD − 2,400,000.5. Ordinal date = year + day of year (001–366).
Frequently asked questions
Why are there two meanings of 'Julian date'?
Astronomers use the continuous Julian Day count; food packaging and the military often use 'Julian date' to mean the ordinal day of the year, like 26045 for 14 Feb 2026.
When does a Julian day start?
At noon UTC, a convention from astronomers who observed at night.
Is this related to the Julian calendar?
Only by name — the Julian Day count was named after Joseph Scaliger's father, Julius, not the calendar.