Date & Time
Julian Date Converter - Gregorian to Julian Day Number
Convert between Gregorian calendar dates and Julian Day Numbers (JDN). Also shows the equivalent Julian calendar date and Modified Julian Date (MJD).
Astronomical applications
Julian Dates are the standard time coordinate throughout astronomy because they eliminate the complexity of the Gregorian calendar (months of varying length, leap years, calendar reforms):
- Calculating the period of variable stars or binary orbits
- Ephemeris computation (predicting positions of planets, comets, and spacecraft)
- Converting between terrestrial, barycentric, and proper time scales
- Archiving and comparing observations taken decades apart
Epoch reference
| Epoch | Julian Date | Calendar equivalent |
|---|---|---|
| Julian Period origin | JD 0.0 | Noon, Jan 1, 4713 BC (proleptic Julian) |
| J2000.0 | JD 2451545.0 | Noon (12:00 TT), Jan 1, 2000 |
| J1950.0 | JD 2433282.5 | Midnight, Jan 1, 1950 |
| Unix epoch | JD 2440587.5 | Midnight, Jan 1, 1970 UTC |
| MJD epoch | JD 2400000.5 | Midnight, Nov 17, 1858 |
What is a Julian Day Number?
The Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC (proleptic Julian calendar). It is widely used in astronomy because it avoids the complexity of months, leap years, and calendar reforms.
Julian Date vs Julian Day Number
The Julian Date (JD) extends the JDN to fractional days. By convention, Julian Date 0.0 starts at noon (12:00 UT) on January 1, 4713 BC, so each integer JDN corresponds to noon on that calendar day.
Modified Julian Date (MJD)
The Modified Julian Date was introduced by the Smithsonian Astrophysical Observatory in 1957 to reduce the large numbers involved. MJD = JD − 2400000.5. The MJD epoch (MJD 0.0) is midnight at the start of November 17, 1858.
Julian calendar vs Gregorian calendar
The Julian calendar (introduced by Julius Caesar in 46 BC) adds a leap year every four years without exception. The Gregorian calendar (1582 reform) skips century leap years unless divisible by 400, making it more accurate. As of 2024, the Julian calendar is 13 days behind the Gregorian calendar.
Frequently asked questions
What is J2000.0?
J2000.0 is the standard astronomical epoch defined as Julian Date 2451545.0, corresponding to January 1.5, 2000 (noon on January 1, 2000 UTC). Many astronomical calculations use J2000.0 as a reference point.
What is the Unix epoch in Julian Days?
The Unix epoch (January 1, 1970 00:00:00 UTC) corresponds to JDN 2440588 and Julian Date 2440587.5 (midnight).