Julian Date Calculator

How to use

1. 1Pick Date to Julian Date or Julian Date to date at the top.
2. 2In Date to Julian Date mode, enter an ISO 8601 UTC date (for example 2026-06-05 or 2026-06-05T12:00:00). Choose Gregorian or Julian as the input calendar.
3. 3Read JD, MJD, RJD, TJD, DJD, day of year, and the weekday side by side, with a Copy button for each value and a Copy summary button for the full block.
4. 4In Julian Date to date mode, paste a plain JD value (such as 2451545.0 for J2000.0) or prefix it with MJD, RJD, TJD, or DJD to use one of the alternative scales.
5. 5Read the matching UTC calendar date (with sub-second precision when present), the human readable date, and the day of year, with copy buttons for each.
6. 6Use the presets (J2000.0 epoch, Unix epoch, MJD epoch, TJD epoch, 1582 calendar reform) and the Use now button to fill the inputs in one click. The live panel keeps current JD, MJD, TJD, and day of year visible while you work.

About this tool

Julian Date Calculator converts a Gregorian (or proleptic Julian) calendar date into the family of Julian Day systems used in astronomy, satellite tracking, FITS file headers, software engineering, surveying, and historical chronology, and runs the inverse from any Julian Day number back to a calendar date. The tool covers all the common scales side by side: Julian Date (JD), the continuous count of days since 12:00 UT on January 1, 4713 BC in the proleptic Julian calendar; Modified Julian Date (MJD), JD minus 2400000.5, which shifts the day boundary from noon to midnight UT and is the scale you see in FITS headers, GPS week computations, orbital mechanics tools, and most modern astronomy software; Reduced Julian Date (RJD), JD minus 2400000, an older form that keeps the noon rollover; Truncated Julian Date (TJD), JD minus 2440000.5, the NASA satellite telemetry and CCSDS time format with an epoch of May 24, 1968; Dublin Julian Date (DJD), JD minus 2415020 with an 1899 epoch used by a few astronomy packages; and Day of Year (DOY, also called ordinal date or in some industries the Julian day), the 1 to 365 or 1 to 366 ordinal that often gets confused with the astronomical JD and is shown here so both meanings live in one place. The arithmetic uses the Meeus formulation from Astronomical Algorithms (chapter 7), so conversions are exact for any proleptic date and the inverse direction automatically switches between the Julian calendar (for JD before 2299161) and the Gregorian calendar (from JD 2299161 onward, the date of the 1582 calendar reform). A toggle lets you force the Julian or Gregorian branch for historical work, and the calculator flags Gregorian dates inside the 1582-10-05 through 1582-10-14 gap that do not exist historically. The tool also reports the weekday computed from the JD, which is the canonical way to get the day of the week for any date in any calendar, and a live now panel shows the current moment as JD, MJD, TJD, and day of year so you can quickly compare against a log entry, an observing run, or a satellite pass. Fractional days encode the time of day, and sub-millisecond precision is preserved in both directions. Useful for amateur and professional astronomers logging observations, students reading textbooks that use JD or MJD, engineers parsing FITS or CCSDS telemetry, surveyors and GIS users working with ephemerides, historians and genealogists computing dates across the 1582 calendar reform, and anyone who has typed today's JD into a paper and wanted to double check. Everything runs in your browser, so the dates you convert never leave your device.

Free to use. Works in your browser. No signup, no login.