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Dublin Julian Date Calculator

Enter a Gregorian date to get its Dublin Julian Date and full Julian Date.

How it is calculated

The tool first computes the Julian Day Number of the calendar date with the Fliegel and Van Flandern integer formula, using the proleptic Gregorian calendar for every year (there is no switch to the Julian calendar in 1582). It then subtracts 0.5, because Julian Days begin at noon and the date is taken at 00:00. This gives JD. Finally DJD = JD − 2415020. Both values are in days and shown with one decimal, so a date at midnight always ends in .5. The date must be a real date written year-month-day with dashes: 2023-02-30 and 01/02/2020 give no result, while 2023-2-5 is accepted. DJD 0 falls at noon on 31 December 1899, so 1900-01-01 at midnight comes out as 0.5 and 1899-12-31 as −0.5.

Worked example

Take 2000-01-01. The Julian Day Number of that date is 2,451,545; at midnight the Julian Date is 2,451,545 − 0.5 = 2,451,544.5. Subtract the Dublin offset: 2,451,544.5 − 2,415,020 = 36,524.5. So the Dublin Julian Date is 36,524.5, which means 36,524 whole days plus half a day after the DJD zero point. As a check, 36,524 days is almost exactly 100 years, matching the span from 1900 to 2000.

Try this example →

Common mistakes

  • Typing the date in day-month-year order or with slashes. The expected format is year-month-day with dashes, and an invalid combination such as 2023-02-30 returns nothing.
  • Expecting a whole number. The date is taken at 00:00, so results end in .5 (the Julian day starts at noon); add the fraction of the day yourself only if you know the time.
  • Mixing up Dublin and Modified Julian Dates. DJD is JD − 2415020, MJD is JD − 2400000.5, so the same date gives different numbers.
  • Using dates before 1582 and expecting the historical Julian calendar. The tool applies the Gregorian rules to every year, so results can differ from sources that use the Julian calendar there.
  • Being surprised by negative values. Dates before 1900-01-01 are earlier than the DJD zero point, so 1899-12-31 gives −0.5.

About this calculator

The Dublin Julian Date (DJD) is an astronomical day-numbering system defined with its epoch at noon UT on 31 December 1899 (1900 January 0.5), adopted at the 1955 International Astronomical Union meeting in Dublin as a more convenient, shorter alternative to the full Julian Date, which counts days from 4713 BCE and produces unwieldy seven-digit numbers for modern dates. It is defined simply as DJD = JD − 2415020, since Julian Day 2415020 corresponds to that epoch.

Because it starts from a recent, round epoch, DJD numbers stay comparatively small and are easier to work with in orbital mechanics, ephemeris tables and some astronomical software that predates or coexists with the more common Julian Date and Modified Julian Date conventions. This calculator converts a Gregorian calendar date into both its full Julian Date and its corresponding Dublin Julian Date.

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Comments (1)

  • Editorial team

    Subtracting two results gives the days between dates. 2026-07-01 returns a DJD of 46,202.5 and 2026-11-04 returns 46,328.5, so the span is 126 days. Both values sit at midnight, so the half-day cancels. As a sanity check, 1900-01-01 should return 0.5. Are you matching an old ephemeris table or an observation log?

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