Julian Date Converter
Convert between calendar dates and Julian Day Numbers — the continuous day count used in astronomy, military, and satellite tracking since 4713 BC.
Current Julian Date (UTC)
2461217.28716
Calendar Date to Julian Date
JD: 2461217.28716
MJD: 61216.78716
Julian Date to Calendar Date
Astronomy
Astronomers use Julian Dates to time celestial events precisely, avoiding calendar complications like leap years and varying month lengths.
Military
Military logistics and communications use Julian date codes (ordinal day of year) for unambiguous date stamping across different time zones.
Satellite Tracking
TLE (Two-Line Element) sets for satellites use Julian dates to specify epoch times, enabling precise orbital calculations.
Modified Julian Date (MJD)
The Modified Julian Date was introduced to provide a more manageable number. It is defined as:
MJD = JD − 2,400,000.5The MJD epoch is November 17, 1858, at 00:00 UTC. Unlike the Julian Date (which starts at noon), MJD starts at midnight, making it more practical for civil timekeeping.
How Julian Day Numbers Work
The Julian Day Number is a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC (proleptic Julian calendar), or November 24, 4714 BC in the proleptic Gregorian calendar. The Julian Day starts at Greenwich noon (12:00 UT).
Conversion Formula (Gregorian to JD):
// If month <= 2: Y = year - 1, M = month + 12
A = floor(Y / 100)
B = 2 - A + floor(A / 4)
JD = floor(365.25 * (Y + 4716)) + floor(30.6001 * (M + 1)) + day + B - 1524.5
This algorithm (from Jean Meeus, Astronomical Algorithms) is valid for all Gregorian calendar dates. The fractional part represents the time of day, where 0.0 = noon UTC, 0.5 = midnight UTC.