Convert between Gregorian calendar dates and Julian Dates (JD) in both directions. Enter a calendar date to get its Julian Day Number, or enter a Julian Date to find the corresponding Gregorian date. The converter also shows the Modified Julian Date (MJD) and day of the week for any input.
Result
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Modified Julian Date (MJD)
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Day of the Week
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Gregorian Date
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Julian Day Number (integer)
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A Julian Date is a continuous count of days and fractions of a day since noon Universal Time on January 1, 4713 BC in the Julian calendar. It is widely used in astronomy and scientific software to represent a moment in time as a single real number, making date arithmetic straightforward.
These are two completely different things. The Julian Calendar is the calendar system introduced by Julius Caesar, which was later replaced by the Gregorian calendar. A Julian Date (JD), on the other hand, is an astronomical time system — a running count of days since January 1, 4713 BC — and has nothing to do with the Julian calendar itself.
The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory to simplify storage and calculation. MJD = JD − 2,400,000.5, which shifts the starting epoch to midnight on November 17, 1858. This makes the numbers smaller and aligns day boundaries with midnight rather than noon.
The noon start was chosen by astronomers in the 19th century so that nighttime observations (which span midnight) all fall within the same Julian Day, avoiding a date change in the middle of a night's work. This convention has been maintained for consistency.
A yyddd date (where yy is the two-digit year and ddd is the day of year, e.g. 26065 for March 6, 2026) is commonly called a 'Julian Date' in mainframe and IT contexts, but it is technically called an ordinal date. It is not the same as the astronomical Julian Day Number used by astronomers.
Select 'Calendar Date → Julian Date', pick your Gregorian date, and the calculator will output the Julian Day Number (JD). The conversion uses a standard astronomical algorithm based on the year, month, and day, accounting for the switch between the Julian and Gregorian calendar systems.
A 64-bit double-precision floating point number can represent a Julian Date to approximately 20 microsecond precision, which is sufficient for nearly all practical and scientific applications. Fractional values of the JD represent the time of day, where 0.5 corresponds to midnight UT.
The Julian Date changes continuously. You can find today's JD by entering today's date in the 'Calendar Date → Julian Date' direction of this converter. As an example, March 6, 2026 corresponds to JD 2461840.5 (at midnight UT).