Julian Date Converter
Convert any calendar date to Julian Date (JD) and Modified Julian Date (MJD).
Also convert JD back to calendar date.
Essential for astronomy.
Julian Date (JD) is a continuous count of days from noon on January 1, 4713 BC (Julian Calendar). It provides a universal timescale for astronomy, eliminating confusion from calendar systems, time zones, and leap years.
Key epoch values:
- J2000.0 = JD 2451545.0 = January 1.5, 2000 (noon on Jan 1, 2000 UTC)
- J1950.0 = JD 2433282.5
- Unix epoch = JD 2440587.5 (January 1, 1970 00:00 UTC)
Modified Julian Date (MJD):
MJD = JD - 2400000.5
MJD starts at midnight rather than noon and is used for modern satellite tracking and space missions. MJD = 0 corresponds to November 17, 1858 (0h UTC).
The algorithm: For a date (Y, M, D) in the Gregorian calendar:
A = floor(Y / 100)
B = 2 - A + floor(A / 4)
JD = floor(365.25 × (Y + 4716)) + floor(30.6001 × (M + 1)) + D + B - 1524.5
For months January and February, use Y−1 and M+12.
Why noon? Julian Dates change at noon (12:00 UTC) rather than midnight. This was historically convenient for astronomers observing throughout the night — the date never changed during a single night’s observations.
J2000 epoch offset: Tells you how many days have elapsed since the J2000.0 epoch. Many orbital mechanics calculations use this as a reference.