What is a Julian Day Number?

The Julian Day Number (JDN) is a continuous count of whole days since noon UTC on 1 January 4713 BC in the proleptic Julian calendar. It is widely used in astronomy and software because it turns dates into a single running integer that is easy to subtract.

What is the difference between JD and MJD?

The Julian Date (JD) adds the fractional day measured from noon UTC, so JD increments at midday. The Modified Julian Date (MJD) is JD minus 2,400,000.5, which shifts the day boundary to midnight and gives smaller, more convenient numbers for modern dates.

Does this work for historical dates?

The calculation uses the proleptic Gregorian calendar for the input date, so it is consistent for any year you enter, but dates before the Gregorian reform of 1582 will differ from contemporary Julian-calendar records.

Julian Day Number Calculator

Julian Day Number, JD and MJD

Convert a Gregorian calendar date (and an optional UTC time) into its Julian Day Number (JDN), the full Julian Date (JD) with a fractional day, and the Modified Julian Date (MJD).

The JDN counts whole days from noon UTC on 1 January 4713 BC, which makes date arithmetic trivial — subtract two JDNs to get the exact number of days between them. JD adds the time-of-day fraction from noon, and MJD = JD − 2,400,000.5. All of it is computed in your browser.