How to use it
- Choose Calendar date to Julian Day or Julian Day to calendar date in Convert.
- Pick the Calendar, Gregorian or Julian, then type the Date and the Time (UT), or type a number in Julian or Modified Julian Date.
- Read the Julian Date (JD), the Julian Day Number (JDN) and the Modified Julian Date (MJD), or the Calendar date that the number names.
- Open How the value is worked out to follow the arithmetic step by step.
What the Julian Day Number counts
The Julian Day Number (JDN) counts each day in one unbroken sequence. JDN 0 is noon UT on Monday 1 January 4713 BC in the Julian calendar, which is 24 November 4714 BC in the Gregorian calendar.
The Julian Date (JD) adds the time of day to the JDN as a fraction. A Julian day begins at noon, so 1 January 2000 at noon UT is JD 2451545.0, and at 18:00 UT it is JD 2451545.25.
The Modified Julian Date (MJD) subtracts 2400000.5, which moves the start of the day to midnight. MJD 0 is 17 November 1858 at 0:00 UT, as the table in the Wikipedia article on the Julian day gives it.
JD = JDN + (hour − 12) / 24 + minute / 1440 + second / 86400
MJD = JD − 2400000.5How a calendar date becomes a JDN
The page uses the integer form of the formula in chapter 7 of Jean Meeus, Astronomical Algorithms (2nd edition, 1998). January and February are counted as months 13 and 14 of the year before. The Gregorian calendar adds a correction B = 2 − A + floor(A / 4), with A = floor(year / 100). The Julian calendar has no such correction.
The day of the week follows from the JDN: with 0 for Sunday, (JDN + 1) mod 7 names it. JDN 0 was a Monday.
The change of calendar in 1582
In the countries that followed Pope Gregory XIII, Thursday 4 October 1582 in the Julian calendar was followed by Friday 15 October 1582 in the Gregorian calendar. The dates 5 to 14 October 1582 were never used in civil life, so ten calendar dates were skipped.
No day was skipped in the Julian Day count. The US Naval Observatory gives 4 October 1582 (Julian) as the day that begins at JD 2299159.5, and 15 October 1582 (Gregorian) as the day that begins at JD 2299160.5.
What this does not cover
- Dynamical Time. A JD here is on Universal Time. Terrestrial Time, which astronomers use for ephemerides, differs from UT by the difference ΔT, and the International Astronomical Union has recommended Terrestrial Time for JD since 1997.
- Local time zones. Subtract your offset from UT first, because the page takes UT.
- Dates before 4713 BC or after 9999 AD. The formulas work outside that range, but the page does not accept them.
Worked example: 1 January 2000 at noon UT
Choose Calendar date to Julian Day, keep the calendar on Gregorian, and type 2000-01-01 with the time 12:00. For the formula, January is month 13 of 1999, so A = 19 and B = 2 − 19 + 4 = −13.
The JDN is floor(365.25 × 6715) + floor(30.6001 × 14) + 1 − 13 − 1524 = 2451545. The day starts at noon, so the JD is 2451545.0, and the MJD is 51544.5.
| Quantity | Value |
|---|---|
| Julian Day Number (JDN) | 2451545 |
| Julian Date (JD) | 2451545.000000 |
| Modified Julian Date (MJD) | 51544.500000 |
Frequently asked questions
What is a Julian Day Number?
It is the count of whole days since noon UT on 1 January 4713 BC in the Julian calendar. Astronomers and computer systems use it because the arithmetic needs no months, leap years or calendar changes.
What is the difference between the Julian Date and the Modified Julian Date?
The Julian Date starts its day at noon UT. The Modified Julian Date, MJD = JD − 2400000.5, starts it at midnight, so MJD 0 is midnight on 17 November 1858.
Why do the dates 5 to 14 October 1582 not exist?
When Catholic countries adopted the Gregorian calendar, Thursday 4 October 1582 was followed by Friday 15 October 1582. Ten calendar dates were skipped, but the Julian Day count did not skip a day.
Why does the page ask for UT and not my local time?
The Julian Date is defined on Universal Time. Convert your local time to UT by subtracting your time zone offset, then type the UT time in the Time field.
Which calendar should I choose for an old date?
Choose the calendar that the source of the date used: the Julian calendar for dates before 15 October 1582 in countries that kept it, and the Gregorian one for later dates in most of the world.
Is the date I type sent to a server?
No. The calculation runs in your browser. Nothing you type is stored or sent to us, and you can use the page without an account.
Sources
- Wikipedia: Julian day (JDN 0, JD and MJD, the Julian and Gregorian formulas, references to Meeus)
- US Naval Observatory: Julian Date Converter, with the 1582 change
- Wikipedia: Gregorian calendar (the reform of 1582 and the day counts)
Figures and text last checked on Tuesday, November 10, 2026.