Two different things called a Julian date
Astronomers use the Julian Date, or JD: a running count of days since noon on 1 January 4713 BC, with the time of day as a decimal. It lets you subtract one date from another without worrying about months, leap years or calendar changes, which is why observatories, satellite trackers and variable-star observers use it. The Modified Julian Date, or MJD, is the same count with 2,400,000.5 taken off, so the numbers are shorter and each day starts at midnight instead of noon.
Everyone else usually means the day of the year: 29 October 2026 is day 302, written 2026-302, or 26302 in the five-figure code stamped on some packaging and used in some workplaces. The bottom section of the tool converts those both ways. Neither has anything to do with the old Julian calendar, which Britain left in 1752.
How to read the answer
The Julian Date changes at 12:00 UTC, not midnight, so the morning and afternoon of one calendar day have different whole numbers. Five decimal places is a resolution of a little under a second. All times are UTC, which is the same as GMT. In British Summer Time, take an hour off the clock time before you convert.
If you compare with a book that gives J2000.0 as JD 2451545.0 at noon on 1 January 2000, that noon is in Terrestrial Time, the steady time scale astronomers use for orbits. It runs about 69 seconds ahead of UTC today, and was 64 seconds ahead in 2000. The tool works in UTC because that is what clocks show, so a figure from an almanac in TT can differ from this one in the fourth decimal place.
How it is worked out
Computers count time in milliseconds since midnight UTC on 1 January 1970, which is Julian Date 2440587.5. Dividing by the 86,400,000 milliseconds in a day and adding that number gives the Julian Date. We checked the results against the worked examples in Jean Meeus's Astronomical Algorithms, the standard reference, including the launch of Sputnik 1 on 4 October 1957, which the book gives as JD 2436116.31.
The tool uses the Gregorian calendar, the one in use today, from its first day: 15 October 1582. Earlier dates need the Julian calendar and a different sum, so the tool refuses them rather than give a wrong answer.
Other days worth a number
The Moon phase finder shows the Moon for any date from 1900 to 2100. For a birthday or an anniversary, the sky on the day you were born explains what a star map of that night shows.
Written by Craig Fearn, Aphelion Prints. Last updated .