Convert between Gregorian calendar dates and Julian Day Numbers (JD) with this Julian Calendar Converter. Enter a year, month, day, and time (UT) to get the corresponding Julian Date — or enter a Julian Date number to convert back to a calendar date. Results include the day of the week and Modified Julian Date (MJD).
Julian Date (JD)
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Modified Julian Date (MJD)
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Calendar Date (UT)
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Day of Week
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Ordinal Date (YYYYDDD)
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A Julian Date is a continuous count of days and fractions since noon Universal Time on January 1, 4713 BC (in the Julian calendar). Julian Dates are widely used in astronomy and scientific software because they provide a single, unambiguous number to represent any moment in time. Almost 2.5 million days have elapsed since the origin date.
The Julian Calendar is a calendar system introduced by Julius Caesar in 46 BC, preceding the Gregorian calendar. A Julian Date (JD), on the other hand, is an astronomical day-counting system with no direct relationship to the Julian Calendar. The naming coincidence often causes confusion — this tool deals with converting between the Julian Day Number system and the Gregorian calendar date.
The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to track the orbit of the Sputnik satellite using smaller numbers that fit in 18 bits. MJD = JD − 2,400,000.5, so it starts from midnight on November 17, 1858. It is commonly used in satellite tracking, astronomy, and geodesy.
The Julian Date system starts at noon (12:00 UT) because astronomers historically observed the sky at night and needed a single date for an entire night's observations. Starting at noon meant a night's observations from dusk to dawn all fell within the same Julian Day, avoiding a date change mid-session.
An ordinal date, sometimes called a 'Julian Date' in mainframe computing, represents a date as the year followed by the sequential day number within that year (1–365 or 1–366 for leap years). For example, March 15, 2025 is 2025074. This format is common in COBOL, mainframe systems, and legacy databases.
Select 'Calendar Date → Julian Date', enter your year, month, day, and optional time in Universal Time (UT), then read the Julian Date result. The calculation uses the standard astronomical formula accounting for the Gregorian calendar reform of October 15, 1582.
Yes. The calculator uses the proleptic Gregorian calendar for dates before October 15, 1582 by applying the Julian calendar formula. Dates on or after October 15, 1582 use the Gregorian formula. This matches the convention used by the U.S. Naval Observatory and most astronomical software.
Julian Dates are based on Universal Time (UT1). A Julian Date fraction of 0.5 corresponds to 0h UT1 (midnight), and a fraction of 0.0 or 1.0 corresponds to noon UT1. When entering times, use Universal Time (approximately equivalent to UTC for most practical purposes).