Julian Date Converter
Free Julian date converter: the astronomical Julian Day and Modified Julian Date of any date and time, the ordinal date YYDDD, and back to a calendar date.
What it does
"Julian date" means two different things. Astronomers use the Julian Day (JD), a continuous count of days since noon on January 1, 4713 BC, which makes it easy to measure time between observations; the Modified Julian Date (MJD) subtracts 2,400,000.5 to start at midnight. In the military, food packaging and manufacturing, a Julian date is the ordinal date: the year followed by the day number, such as 26301 for October 28, 2026. This converter gives both and converts them back to a calendar date.
How to use it
- Choose a date and, for the astronomical Julian Day, the time in UT.
- See the Julian Day, the MJD and the ordinal dates.
- To convert back, type a Julian Day or an ordinal date such as 26301.
The arithmetic runs in your browser. Nothing you type is sent anywhere and nothing is stored, so you can use these with real numbers.
Frequently asked questions
Why does the Julian Day start at noon?
It was designed for astronomers, who observe at night: starting the day at noon keeps a whole night of observations on the same day number. JD 2451545.0 is noon on January 1, 2000.
How do I read a Julian date on food packaging?
It is usually the last digit or two of the year followed by the day of the year: 26301 or 6301 means day 301 of 2026, October 28. Check the manufacturer's format, since some put the day first.
Is it related to the Julian calendar?
Only in name. The Julian calendar is the one created by Julius Caesar and replaced by the Gregorian calendar; the Julian Day count was proposed by Joseph Scaliger in 1583 and works with any calendar.
What is the MJD used for?
The Modified Julian Date, JD − 2,400,000.5, gives smaller numbers that start at midnight. It is used for satellites, space missions and scientific data; MJD 0 was November 17, 1858.
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