Convert between Gregorian dates, Julian Day Numbers (JDN), Modified Julian Dates (MJD), and the historic Julian Calendar. Essential for astronomy, history, and archival research.
Reference
A continuous count of days since the start of the Julian Period — noon on January 1, 4713 BC (proleptic Julian calendar). Widely used in astronomy to avoid calendar ambiguity. Today's JDN is around .
A compact variant: MJD = JDN − 2400000.5. Starts at midnight rather than noon, and the epoch is November 17, 1858. Preferred by satellite and space operations for its smaller numbers.
Introduced by Julius Caesar in 46 BC, it assumed a 365.25-day year. By 1582 it had drifted ~10 days behind the solar year. Pope Gregory XIII replaced it with the Gregorian calendar on October 15, 1582. Historic dates before that are given in "Old Style" (O.S.).
The Julian calendar currently runs 13 days behind the Gregorian calendar (as of the 20th–21st century). The gap grows by 3 days every 400 years, next increasing to 14 days in 2100.