Julian date converter
Enter a date and time in UTC to get the astronomical Julian Day, its day number and the Modified Julian Date, along with the YYDDD form used on packaging and in mainframe systems. Or enter a number and see the date.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
Date to Julian date. Enter a date and a UTC time (or press the button for the current time). You get the JD to five decimals (about one second), the Julian Day Number, the Modified Julian Date and the day-of-year date.
Julian date to date. Choose what kind of number you have: a Julian Day, a Modified Julian Date, or a YYDDD or YYYYDDD day-of-year date. The date and the weekday appear below. A five-digit YYDDD is read with the usual rule for two-digit years: 69 to 99 mean 1969 to 1999, 00 to 68 mean 2000 to 2068.
Two things called “Julian date”
- Astronomical. A continuous count of days. The Julian Day starts at noon, so 00:00 UTC on 3 November 2026 is JD 2461347.5 and noon is JD 2461348.0. Spacecraft logs, satellite data and astronomy software use it or the Modified Julian Date.
- Day of the year. Year and day number, as in 26307. It is easy to sort and compare, which is why it appears on lot codes and in older business systems.
How it works
The Julian Day follows from the Unix time: JD = days since 1970-01-01 + 2440587.5, because the Unix epoch is JD 2440587.5. The MJD is JD − 2400000.5. The JDN is the integer part of JD + 0.5.
Worked example
3 November 2026, 12:00 UTC:
- 9,803 days after 00:00 UTC on 1 January 2000 (JD 2451544.5, since J2000.0 at noon is 2451545.0);
- JD = 2451544.5 + 9803 + 0.5 = 2461348.0;
- MJD = 2461348.0 − 2400000.5 = 61347.5;
- the day of the year is 307, so the YYDDD form is 26307.
Frequently asked questions
What is a Julian Day?
The Julian Day (JD) is the number of days, with a fraction, since noon UTC on 1 January 4713 BC in the Julian calendar. Astronomers use it because it is one continuous count with no months or leap years. JD 2451545.0 is noon UTC on 1 January 2000.
What is the difference between JD, JDN and MJD?
JD is the full count with a fraction of a day. The JDN is the whole number for a calendar day, the JD at its noon, so all of 3 November 2026 has the JDN 2461348. The Modified Julian Date is JD minus 2400000.5, which makes the day start at midnight and keeps the numbers short; MJD 0 is midnight at the start of 17 November 1858.
What is the Julian date in YYDDD form?
In manufacturing, food packaging, logistics and many mainframe systems, a Julian date means the year followed by the day of the year. 3 November 2026 is day 307, written 26307 (YYDDD) or 2026307 (YYYYDDD). It has nothing to do with the Julian calendar or the astronomical count.
Which calendar does the converter use?
Dates are in UTC and in the Gregorian calendar, extended backwards (proleptic) for the years before 1582. Years from 1000 to 9999 are accepted.