Convert any moment to and from the Julian Date family — the day-numbering system astronomers, satellite operators, and mainframe shops have used since 1583. The integer part is the Julian Day Number; the fractional part is the time of day measured from noon UT. Modified JD (MJD) subtracts 2 400 000.5 to give a smaller number whose integer roll-over is midnight.
| Variant | Offset from JD | Epoch (JD = 0 or 0.0 there) | Where you see it |
|---|---|---|---|
| Julian Date (JD) | 0 | −4713-01-01 12:00 UT (Julian cal.) | Astronomy ephemerides |
| Modified JD (MJD) | JD − 2400000.5 | 1858-11-17 00:00 UT | IERS, GPS, satellite telemetry |
| Reduced JD (RJD) | JD − 2400000 | 1858-11-16 12:00 UT | Older astronomy literature |
| Truncated JD (TJD) | JD − 2440000.5 | 1968-05-24 00:00 UT | NASA, code that needs 4-digit JD |
| Dublin JD (DJD) | JD − 2415020 | 1899-12-31 12:00 UT | IAU, older almanacs |
| J2000 days | JD − 2451545.0 | 2000-01-01 12:00 UT | Modern astrodynamics, SGP4 |
| ANSI Date | JDN − 2305814 | 1601-01-01 00:00 (proleptic Greg.) | COBOL, mainframes, Windows |
| Unix time / 86400 | JD − 2440587.5 | 1970-01-01 00:00 UT | POSIX |
The classic Julian Date starts a new day at noon Universal Time — that's why JD 0 begins at noon on 1 January 4713 BC in the proleptic Julian calendar. MJD shifts the start to midnight by subtracting 2 400 000.5, which is what made it the practical choice for engineering. Conversion uses Fliegel & Van Flandern's 1968 integer algorithm. Before 1582-10-15 the Gregorian reform hadn't happened yet, so dates differ from the proleptic extension. Tick the proleptic Gregorian box if you want to treat early dates as if Pope Gregory's calendar had always been in effect — most astronomy software does. Note this tool does not apply leap seconds: it treats UT as a uniform scale equal to UTC for practical purposes.