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Modified Julian Date Calculator

Enter a calendar date to get its Modified Julian Date and full Julian Date.

How it is calculated

The date is typed as YYYY-MM-DD and is read as a proleptic Gregorian calendar date, even for years before the Gregorian reform. A standard integer algorithm turns year, month and day into the Julian Day Number, which counts whole days and refers to noon. Because the tool evaluates midnight at the start of the day (00:00 UT), the Julian Date is JD = JDN − 0.5. The Modified Julian Date is MJD = JD − 2,400,000.5, which makes it a whole number at midnight. The tool also names the weekday. Impossible dates such as 30 February, or any format other than year-month-day, return no result, and there is no time-of-day or time zone input.

Worked example

Take 1 January 2000. The algorithm gives a Julian Day Number of 2,451,545, so at 00:00 UT the Julian Date is 2,451,545 − 0.5 = 2,451,544.5. Subtracting 2,400,000.5 gives MJD = 51,544.0, which is the main result shown as 51,544.0. The weekday is Saturday. As a check, 17 November 1858 gives MJD 0.0, the zero point of the scale, so 51,544 days separate it from 1 January 2000.

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Common mistakes

  • Typing the date as 01/01/2000 or 1-1-2000. Only the form year-month-day with four-digit year is accepted, so write 2000-01-01.
  • Confusing JD and MJD. JD changes at noon and is around 2.4 million, while MJD changes at midnight; when you mix the two the day is out by half a day and by 2,400,000.5.
  • Expecting a time of day. The result is for 00:00 UT of the date; for an observation at 18:00 UT add 0.75 to the MJD yourself.
  • Entering a local date that is already the next day in UT. If your local time is ahead of UT, the UT date can be the day before, so convert to UT first.
  • Using the result for historical dates before 1582 as a Julian-calendar date. The tool applies Gregorian rules to every year, so it will not match sources that use the Julian calendar for those dates.

About this calculator

The Julian Date (JD) is a continuous day count astronomers use instead of the calendar, counting from noon UT on January 1, 4713 BC — a system chosen precisely so it never resets and never has to deal with months, leap years, or calendar reforms. The Modified Julian Date (MJD) is JD minus 2,400,000.5, which both shortens the huge JD number and shifts the day boundary to midnight, making MJD = 0 correspond to November 17, 1858.

MJD is the standard timestamp in astronomy, satellite operations, and GPS/GNSS systems, where it's far more convenient than calendar dates for computing time intervals and orbital calculations — it's used, for instance, in satellite ephemeris data and by observatories logging observation times.

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  • Editorial team

    MJD counts days from midnight on 17 November 1858, so that date returns 0.0, and it keeps numbers short for logs and observation records. The tool evaluates 00:00 UT only. If your event happened later in the day, add the fraction yourself: 18:00 adds 0.75 and 06:00 adds 0.25. Which time scale do your records use, UTC or something else?

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