Julian Date Converter

Enter a date and time in UT and press Convert, or switch direction to turn a Julian Date, Modified Julian Date, or Unix timestamp back into a calendar date. All the related day counts are shown at once.

Try an example

About this Julian Date converter

Astronomers count time as a single running number of days, the Julian Date, starting at noon Universal Time on 1 January 4713 BC in the Julian calendar. It avoids months, leap years, and calendar reforms, which makes intervals easy to compute: J2000.0, the reference epoch for modern star charts, is JD 2451545.0. Variants make the numbers shorter, such as the Modified Julian Date (JD minus 2400000.5, starting at midnight) used by satellites and observatories, and the Reduced and Truncated Julian Dates. This converter uses Jean Meeus’s algorithms, works with dates in either the Gregorian or the older Julian calendar, and also gives the Unix time, day of the year, and weekday.

How to convert a Julian Date

  1. Choose the direction. Date → Julian turns a calendar date into a Julian Date; Julian → Date does the reverse for JD, MJD, or Unix time.
  2. Enter the values. Type the date and time in Universal Time and pick the calendar, or enter the number to convert. Press an example to try it.
  3. Press Convert. Copy any of the results, from the Julian Day Number to the Gregorian and Julian calendar dates.

Frequently asked questions

Why does the Julian Date start at noon?

It was designed for astronomers, who observe at night. Starting days at noon UT meant a whole night of observations had the same day number.

What is the difference between JD and JDN?

The Julian Date includes a fraction for the time of day. The Julian Day Number is the whole-number day that begins at noon, so it is JD rounded down after adding 0.5 days.

What is the Modified Julian Date?

MJD = JD − 2400000.5. It starts at midnight and uses smaller numbers; MJD 0 is 17 November 1858.

Is this the same as the “Julian date” on food packaging?

No. On packaging and in manufacturing, “Julian date” usually means the year and day of the year, such as 26277. That form is shown in the results as YYYYDDD.

When should I choose the Julian calendar?

For historical dates written in the Julian calendar, which most of Europe used before October 1582 and some countries until the 1920s. The day after 4 October 1582 (Julian) was 15 October 1582 (Gregorian).