Julian Date Converter

About the Julian Date Converter

Turn any date and time into the astronomical Julian Date (JD) and Modified Julian Date (MJD), or type a JD, MJD or Unix time and get the calendar date and time back. The same page reads the ordinal "Julian date" codes printed on food packs, military paperwork and production lots (YYDDD such as 26278, YYYYDDD such as 2026278, YDDD, CYYDDD and plain day numbers) and turns any date into them.

It also converts between the Julian and Gregorian calendars: proleptically, or the way history did it. Choose a country’s switch-over (Rome’s in October 1582, Britain’s in September 1752, Russia’s in 1918 and others) and earlier dates are read as Julian (Old Style), later ones as Gregorian (New Style), with the skipped days refused. The arithmetic is exact to the millisecond (JD and MJD are shown to six decimals, about a tenth of a second) and runs in your browser.

Common uses

• Astronomy and satellite work: JD, MJD and RJD for observation logs, ephemerides and variable-star timings.
• Reading the pack or production date on food and drink, egg cartons and military supply documents printed as YYDDD, YDDD or a day number.
• Genealogy and history: Old Style dates from British records before 1752, Russian dates before 1918 and other pre-reform dates.
• Programming and data work: checking a Julian Day, MJD, Rata Die or Unix timestamp that your code produces, and ISO 8601 ordinal dates (YYYY-DDD).

How each value is defined

• Julian Date (JD): days and fractions counted from noon Universal Time on 1 January 4713 BC in the Julian calendar, as the US Naval Observatory defines it. JD changes at noon UT, so 2451545.0 is noon on 1 January 2000.
• Julian Day Number (JDN): the whole JD at noon of a calendar date, the number most date software means by "Julian day".
• Modified Julian Date (MJD) = JD − 2400000.5. It starts at midnight and has shorter numbers; MJD 0 is the start of 17 November 1858.
• Reduced Julian Date (RJD) = JD − 2400000, common in variable-star work. Days since J2000.0 = JD − 2451545.0, the starting point of most modern astronomical formulas.
• Unix time = (JD − 2440587.5) × 86,400 seconds. Like Unix clocks, it ignores leap seconds.
• Rata Die numbers 1 January AD 1 (Gregorian) as day 1; the Lilian day number numbers 15 October 1582, the first Gregorian day, as day 1.

The calendar arithmetic uses the exact integer day-number algorithms of Fliegel and Van Flandern (as given by Richards), checked against the worked examples in Meeus’s Astronomical Algorithms.

Julian and Gregorian calendars, and when each country switched

The Julian calendar adds a leap day every fourth year. The Gregorian reform drops three leap days every 400 years (1700, 1800 and 1900 are not leap years, 2000 is), so the Julian calendar falls one day further behind in each such century: 10 days in 1582–1700, 11 in 1700–1800, 12 in 1800–1900 and 13 in 1900–2100. The weekday never changes across a switch.

Choose Historical and the country to read dates the way they were written there:

• Italy, Spain, Portugal and Poland–Lithuania (the papal bull Inter gravissimas): Thursday 4 October 1582 was followed by Friday 15 October 1582.
• France: Sunday 9 December 1582 was followed by Monday 20 December 1582.
• Denmark–Norway and the Protestant German states: Sunday 18 February 1700 was followed by Monday 1 March 1700.
• Great Britain and its colonies, by the Calendar (New Style) Act 1750: Wednesday 2 September 1752 was followed by Thursday 14 September 1752.
• Bulgaria: 31 March 1916 was followed by 14 April 1916. Russia: 31 January 1918 was followed by 14 February 1918. Romania: 31 March 1919 was followed by 14 April 1919. Greece: 15 February 1923 was followed by 1 March 1923.

For anywhere else choose Custom and enter the last Julian day there; the next day is the first Gregorian one.

Old Style years that began on 25 March

In England, Wales and the British colonies the legal year began on 25 March (Lady Day) up to 1751, so a date between 1 January and 24 March carries the previous year’s number. George Washington was born on 11 February 1731 by the calendar of the day, often written 1731/32, which is 11 February 1732 in the Julian calendar with years starting in January and 22 February 1732 in the Gregorian calendar. Tick Old Style year to read dates written that way. The Act shortened 1751 to end on 31 December, so 1 January to 24 March 1751 never occurred in Old Style dating, and 1752 began on 1 January.

Ordinal "Julian date" codes

Packs, egg cartons, military documents and production lots often print the date as the day of the year. It is called a Julian date, although it has nothing to do with the Julian Day or the Julian calendar:

• YYDDD: 26278 is day 278 of 2026 (5 October).
• YYYYDDD: 2026278 is the same day; ISO 8601 writes it 2026-278.
• YDDD: 6278 uses only the last digit of the year.
• DDD: 278 has no year at all.
• CYYDDD: 126278 puts a century digit before YYDDD (0 = 1900s, 1 = 2000s), as JD Edwards and some other business systems store dates.

Day 001 is 1 January. In a leap year 29 February is day 060 and 31 December day 366. A code with a one- or two-digit year does not say which decade or century it means, so the converter takes the matching year nearest the reference year (this year unless you change it) and lists the other candidates.

Limitations

• Julian Dates are worked out in the time scale you enter: Universal Time, or local time with its UTC offset. Almanacs often give JD in Terrestrial Time (TT), which runs about a minute ahead of UT; ΔT and leap seconds are not applied.
• Dates from 1 January 4713 BC (JD 0 is its noon) to 31 December 9999 are supported. In ISO 8601 output, years are astronomical: 1 BC is year 0000 and 44 BC is −0043.
• The historical switches are for each country as a whole. Some regions changed at other times (parts of the Netherlands, Switzerland and Germany, for example), and Sweden’s calendar of 1700–1712 is not modelled; enter any other switch with Custom.
• Ordinal codes are read in the Gregorian calendar. Manufacturers’ lot codes may add letters, line or plant numbers around the date: type only the date digits, and check the manufacturer’s own key when in doubt.

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