Julian Day Number (JDN) Calculator
Convert any Gregorian calendar date to its Julian Day Number (JDN) and Modified Julian Date (MJD), the continuous day count used in astronomy.
Results
What is it?
For entertainment purposes only. The Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period (January 1, 4713 BC in the proleptic Julian calendar). It is the standard timekeeping system used in astronomy to avoid the complexity of months and years. The Modified Julian Date (MJD) is JDN - 2,400,000.5 and starts from midnight of November 17, 1858 CE � a more compact representation.
How to use
Enter the year, month, and day in the Gregorian calendar. The calculator uses the standard astronomical formula to compute the exact Julian Day Number at noon of that date. The MJD is also provided for use with modern astronomical software.
Example scenario
January 1, 2000 (J2000.0 epoch, the standard reference epoch for modern astronomy): JDN = 2,451,545. This is the reference point for many astronomical calculations. The date July 1, 2024 = JDN 2,460,492 � meaning over 2.46 million days have passed since 4713 BC.
Pro tip
To find the number of days between any two dates, simply subtract their JDNs. For example, the number of days from the Moon landing (July 20, 1969) to today can be calculated precisely without worrying about leap years or month lengths. JDNs are also used in spreadsheet formulas and database date arithmetic as an intermediary format.