Julian date converter.

Two different things called “Julian date”

Astronomy: the Julian Date (JD) is a continuous count of days and fractions of a day since noon Universal Time on 1 January 4713 BC (Julian calendar). Midnight UT is therefore a fraction of .5. This tool produces that number. Ordinal date: in military, logistics and food-packaging use, “Julian date” often means the day of the year, such as 2026-273 or just 273 for 30 September 2026. That is not the astronomer’s JD. The result panel shows both, labelled separately: the ordinal date as YYYY-DDD (the ISO 8601 ordinal form) and the day of the year.

What each result means

Julian Date (JD) includes the time of day. Julian Day Number (JDN) is the whole-day number assigned to a calendar date: the JD of noon on that date, so 1 January 2000 is JDN 2451545 (strictly, JDN 2451545 runs noon to noon, so the morning hours of 1 January 2000 fall in the day number below it). Modified Julian Date (MJD) is JD − 2400000.5, which makes it start at midnight and keeps the numbers short; MJD 0 is 17 November 1858. Reference points you can check: JD 2451545.0 is 2000-01-01 12:00 (the J2000.0 epoch is defined in Terrestrial Time, TT), and JD 2440587.5 is the Unix epoch, 1970-01-01 00:00 UTC.

Gregorian, Julian and the 1582 switch

The default (historical) setting uses the Julian calendar up to 4 October 1582 and the Gregorian calendar from 15 October 1582, as in Meeus’ algorithm; the ten days between never existed, so entering them is rejected. Pick a proleptic option to extend one calendar across all dates. Real countries adopted the Gregorian calendar at different times (Britain and its colonies in 1752, for instance), so historical dates outside Catholic Europe may need the Julian setting. Years use astronomical numbering, where 1 BC is year 0. For 1582 the ordinal day counts days actually elapsed since 1 January, so 15 October 1582 is day 278.

Method, sources and limits

The conversion is the integer algorithm from Jean Meeus, Astronomical Algorithms, 2nd ed., chapter 7, using only floor arithmetic, and was cross-checked against independent Fliegel–Van Flandern and Richards integer algorithms and JavaScript’s own date arithmetic. Reference values: US Naval Observatory, Julian Date definition; USNO worked example: 1877-08-11 07:30 UT1 = JD 2406842.8125; NASA HEASARC: MJD = JD − 2400000.5; Julian day (J2000.0, Unix epoch, MJD epoch). Sources checked 30 September 2026. Supported range is JD 0 (4713 BC) to the year 9999. Input times are treated as UT and are not converted between UT, UTC and TT, and leap seconds are not applied; results are shown to 8 decimals (about a millisecond).

All calculations run locally in your browser. No date is uploaded or stored.