All Tools VerseThe Julian Day Number (JDN) is a count of whole days since noon UT on 1 January 4713 BC in the Julian calendar. Astronomers use it so that the gap between any two dates is a plain subtraction. The Julian Date (JD) adds the fraction of the day since noon, so 1 January 2000 at 12:00 UT is JD 2451545.0. The Modified Julian Date is MJD = JD minus 2,400,000.5, which starts each day at midnight and keeps the numbers shorter.
In this guide
JD, JDN and MJD side by side
| Name | What it is | Day starts at | Example: 8 Nov 2026, 18:00 UT |
|---|---|---|---|
| Julian Date (JD) | Days and fraction since the start of the count | Noon UT | 2461353.25 |
| Julian Day Number (JDN) | The whole-number part, one number per day | Noon UT | 2461353 |
| Modified Julian Date (MJD) | JD minus 2,400,000.5 | Midnight UT | 61352.75 |
The day count builds on the Julian Period, which the scholar Joseph Scaliger described in 1583. The noon start suited astronomers working at night: a whole night of observing falls on one day number. MJD day zero is 17 November 1858 at 00:00 UT.
Reference dates
Every value below was computed with two independent methods (the Meeus algorithm and a day count from the Gregorian calendar) and they agree.
| Date and time (UT) | JD | MJD |
|---|---|---|
| 1858-11-17 00:00 | 2400000.5 | 0 |
| 1970-01-01 00:00 (Unix epoch) | 2440587.5 | 40587 |
| 2000-01-01 00:00 | 2451544.5 | 51544 |
| 2000-01-01 12:00 (J2000.0) | 2451545.0 | 51544.5 |
| 2026-11-08 00:00 | 2461352.5 | 61352 |
| 2026-11-08 12:00 | 2461353.0 | 61352.5 |
Notice that midnight always ends in .5 and noon is a whole number. That is the noon start at work. To get these numbers for any date, use the date to Julian Day converter. It shows JD, JDN, MJD and days since J2000 together. To go the other way, the Julian Day to date converter turns a JD back into a calendar date and time.
How to calculate the JDN
For a date in the Gregorian calendar, this integer formula by Fliegel and Van Flandern (1968) gives the JDN. All divisions drop the remainder.
a = (14 - month) / 12 y = year + 4800 - a m = month + 12 × a - 3 JDN = day + (153 × m + 2) / 5 + 365 × y + y / 4 - y / 100 + y / 400 - 32045
For 8 November 2026: a = 0, y = 6826, m = 8, and JDN = 2461353. To get the JD for a time of day, add (hour minus 12) ÷ 24, plus minutes and seconds as fractions. At 18:00 that is 2461353 + 6/24 = 2461353.25.
The Gregorian calendar started on 15 October 1582. The day before was 4 October 1582 in the Julian calendar. Their JDs are 2299160.5 and 2299159.5 at midnight, one day apart, even though the dates jump by ten. For older dates, decide which calendar your source uses before you convert.
What the numbers are good for
- Days between two dates. JD for 2026-11-08 minus JD for 2000-01-01 is 9,808 days. No month lengths or leap years to think about.
- Day of the week. (JDN + 1) mod 7 gives 0 for Sunday, 1 for Monday, and so on. For 8 November 2026, (2461353 + 1) mod 7 is 0, a Sunday.
- Converting Unix time. JD = Unix seconds ÷ 86,400 + 2440587.5. See our guide to Unix epoch time for the other half of that conversion.
- Storing precise times. A JD near 2.46 million, stored as a standard 64-bit float, can only resolve about 40 microseconds. The same moment as an MJD resolves to under one microsecond. That is one reason MJD is common in satellite and telescope data.
Common mistakes
- Forgetting the noon start. A date entered at 00:00 gives a JD ending in .5. If you then round down, you get the previous day’s JDN. The JDN of a calendar date is the JD at noon that day.
- Using local time. JD is defined in UT. 18:00 in New York in November is 23:00 UT, five hours later.
- Confusing it with the “Julian date” on packaging. Food plants and the military often call the day of the year a Julian date. 8 November 2026 is day 312, written 26312 or 2026312. That is a different thing from the astronomical JD.
- Mixing up MJD and JD minus 2,400,000. The extra 0.5 is easy to drop, and it shifts every result by 12 hours.
For more date and time converters, see our list of the best free time and date tools.
Frequently asked questions
What is the Julian date today?
For 8 November 2026, the JDN is 2461353. At 00:00 UT the JD is 2461352.5, and at 12:00 UT it is 2461353.0. The converter starts with the current date filled in.
What is the difference between JD and MJD?
MJD is JD minus 2,400,000.5. It drops the first two digits and moves the start of the day from noon to midnight.
Why does the Julian Day start at noon?
Astronomers work at night. With the day starting at noon, a whole night of observations from Europe falls on one day number instead of being split at midnight.
Is the Julian Day related to the Julian calendar?
Only by name and starting point. The count began on a date in the Julian calendar, but it works with any calendar. Most software today converts from Gregorian dates.