World Calendars
Julian Day number today
Astronomers and computers often number days, or seconds, from one fixed starting point instead of using months and years. The Julian Day counts from 4713 BC; the others here start in 1858, AD 1 and 1970.
2 461 356
at 12:00 UTC
- In the Gregorian calendar
- Wednesday 11 November 2026
- This day began
- At 12:00 UTC on Wednesday 11 November 2026
- This day ends
- At 12:00 UTC on Thursday 12 November 2026
- Modified Julian Day
- 61 355 (at 00:00 UTC)
- Rata Die
- 739 931
- Unix time
- 1794355200 (at 00:00 UTC)
- Count began
- 24 November 4714 BC, at noon UTC, day 0 (Gregorian calendar counted backwards)
- Who uses it
- Astronomers (Julian Day), satellite trackers (Modified Julian Day), computer systems (Unix time)
- How it works
- Each day gets one number, so the gap between dates is one subtraction
- Day begins
- At noon for the Julian Day, so a whole night has one date
- Unusual detail
- Unix time ignores leap seconds; 32-bit counters overflow on 19 January 2038
How the day counts work
A day count gives every day a single number, so the time between two dates is one subtraction. The four counts below differ in their starting point, called the epoch, and in whether they count days or seconds.
| Count | Epoch (zero) | Counts | Main users |
|---|---|---|---|
| Julian Day (JD) | Noon UT, 1 January 4713 BC (Julian calendar) | Days | Astronomers |
| Modified Julian Day (MJD) | Midnight, 17 November 1858 | Days (JD - 2,400,000.5) | Satellite tracking |
| Rata Die (RD) | Midnight, 31 December 1 BC (Gregorian); day 1 is 1 January AD 1 | Days (JD - 1,721,424.5) | Calendar software (Python, Go, Rexx) |
| Unix time | 00:00:00 UTC, 1 January 1970 | Seconds | Operating systems, programming languages, databases |
The Julian Day epoch is 24 November 4714 BC on the proleptic Gregorian calendar. Julian days begin at noon because, when John Herschel recommended them, the astronomer's day began at noon, which kept a whole night's observations under one date. Astronomers moved to days starting at midnight in 1925, but Julian days kept noon. The Julian day number (JDN) is the whole number of the day; the Julian date (JD) adds the fraction of a day since the last noon, as a decimal.
Unix time ignores leap seconds. Every day has exactly 86,400 seconds, and after a positive leap second the same Unix number is used twice. A signed 32-bit counter overflows after 03:14:07 UTC on 19 January 2038.
Variants and the method used here
The number at the top is the Julian day number of the date shown, which is the JD at 12:00 UTC on that day. Before noon UTC the running Julian date is still below this number, because the Julian day starts at noon. From the same day number:
- MJD at 00:00 UTC is the JDN minus 2,400,001.
- Rata Die is the JDN minus 1,721,425, the form of RD that labels each day by its noon.
- Unix time at 00:00 UTC is the JDN minus 2,440,588, times 86,400.
Dates are entered on the Gregorian calendar; for dates before it took effect on 15 October 1582, that means the proleptic Gregorian calendar, which extends its rules backwards. Rata Die can depend on the time zone, because it starts at local midnight.
A short history
In 1583 Joseph Scaliger proposed the Julian period of 7,980 years, the product of a 28-year solar cycle, the 19-year lunar cycle and the 15-year indiction. Ludwig Ideler first used Julian days in his Handbuch of 1825, and John Herschel developed them for astronomy in his Outlines of Astronomy of 1849. Norman Pogson brought Julian dates into the study of variable stars in 1860.
The Smithsonian Astrophysical Observatory introduced the Modified Julian Day in 1957 to record the orbit of Sputnik on an IBM 704 computer. Rata Die is used in the book Calendrical Calculations by Edward Reingold and Nachum Dershowitz. The earliest versions of Unix counted sixtieths of a second and tried epochs in 1971 and 1972 before settling on 1 January 1970, picked by Unix engineers as a convenient date.
Sources
How this site calculates the date: Days since noon UTC on 1 January 4713 BC (Julian). The number shown is the JD at noon UTC of this day.
Dates checked every month against US Naval Observatory Julian Date service.