Julian Day Number calculator: Julian Date for astronomy
The Julian Date is a continuous count of days and fractions since noon Universal Time on 1 January 4713 BC. At 00:00 UTC on 23 October 2026 it is 2461336.5. Enter a date and time in UTC to get the Julian Date, or a Julian Date to get the calendar date.
What the Julian Date counts
The Julian Date (JD) numbers every day in one unbroken sequence, starting at noon Universal Time on 1 January 4713 BC in the proleptic Julian calendar. That moment is JD 0.0. Months, years and calendar reforms do not interrupt the count, so the days between two events are a plain subtraction.
The fraction after the decimal point is the time since noon UT. Midnight UT is .5, 6 p.m. is .25 and 6 a.m. is .75 of the previous day number. On 23 October 2026, for example:
| Time (UTC) | Julian Date | Modified Julian Date |
|---|---|---|
| 00:00 | 2461336.5 | 61336.0 |
| 06:00 | 2461336.75 | 61336.25 |
| 12:00 | 2461337.0 | 61336.5 |
| 18:00 | 2461337.25 | 61336.75 |
The Julian Day Number (JDN) is the integer JD at noon of a civil date: 2461337 for 23 October 2026. The day of the week follows from it: (JDN + 1) mod 7 gives 0 for Sunday, so 2461337 gives 5, a Friday.
Why the day starts at noon
The count was designed for astronomers, who observe at night. With the day starting at noon UT, a whole night of observations in Europe falls under one day number, and no date changes in the middle of it. The price is that civil midnight is always a half day, which is the most common source of off-by-0.5 errors.
Dates before 15 October 1582 are counted in the Julian calendar and later dates in the Gregorian calendar, as the US Naval Observatory does. That is why JD 2299160.0 is 4 October 1582 (Julian) and JD 2299161.0, one day later, is 15 October 1582 (Gregorian). The Julian to Gregorian converter explains the ten missing days.
Modified Julian Date and other variants
Seven-digit numbers with a half-day offset are awkward, so several shorter forms are in use:
- Modified Julian Date (MJD) = JD - 2400000.5. It starts at midnight; MJD 0 is 17 November 1858 at 00:00. Satellite data, geodesy and many observatories use it.
- Reduced Julian Date (RJD) = JD - 2400000, still noon-based.
- Truncated Julian Date (TJD), used by NASA, = JD - 2440000.5.
- Unix time is a day count too, in seconds: Unix time = (JD - 2440587.5) x 86400. JD 2440587.5 is 1 January 1970 at 00:00 UTC; the Unix timestamp converter covers it in detail.
Converting a date to JD by hand
Jean Meeus gives a short method in Astronomical Algorithms (chapter 7). For year Y, month M and day D with the time as a fraction of D:
- If M is 1 or 2, use Y - 1 and M + 12 (January and February count as months 13 and 14 of the previous year).
- For a Gregorian date, set A = INT(Y / 100) and B = 2 - A + INT(A / 4). For a Julian calendar date, B = 0.
- JD = INT(365.25 x (Y + 4716)) + INT(30.6001 x (M + 1)) + D + B - 1524.5.
For 23 October 2026 at 00:00: A = 20 and B = -13. INT(365.25 x 6742) = 2462515 and INT(30.6001 x 11) = 336. Then 2462515 + 336 + 23 - 13 - 1524.5 = 2461336.5. The converter uses the equivalent algorithm of Fliegel and Van Flandern, which works in whole days and avoids floating-point rounding.
UT and TT: which clock the JD uses
A Julian Date by itself does not say which time scale it counts in. Civil uses and this converter take UT, in practice UTC. Ephemerides and orbit calculations usually use Terrestrial Time (TT), which runs ahead of UTC by 32.184 seconds plus the leap seconds so far, 69.184 seconds since 2017.
The standard epoch J2000.0 is defined in TT: JD 2451545.0 is 1 January 2000 at 12:00 TT, which was 11:58:55.816 UTC. The difference only matters when you need seconds. Software written for astronomy often labels its values JD(TT) or JD(UTC) for this reason.
Julian Day, Julian date and the Julian calendar
Three different things share the name, and search results often mix them up:
- The Julian Day on this page is a day count from 4713 BC, used in astronomy and science software.
- A Julian date code such as 26296 is the year followed by the day of the year, used on packaging, in logistics and in mainframes. The Julian date converter reads them.
- The Julian calendar is the calendar introduced under Julius Caesar, which some Orthodox churches still use. Today it runs 13 days behind the Gregorian calendar: 23 October 2026 is 10 October in the Julian calendar.
The name of the day count comes from the Julian period of 7,980 years that Joseph Scaliger proposed in 1583, which is where the 4713 BC starting point comes from.
Questions
What is today's Julian Day Number?
The converter above shows it. For example, for 23 October 2026 the Julian Day Number is 2461337, the Julian Date at noon UT. From midnight to noon UT that day the Julian Date runs from 2461336.5 to 2461337.0.
Why does the Julian Day start at noon?
The count was set up for astronomers, who observe at night. Starting the day at noon UT keeps a whole night of observations under one day number, so midnight UT is always a .5 value.
What is the Modified Julian Date?
MJD = JD - 2400000.5. It starts at midnight instead of noon and gives smaller numbers: MJD 0 is 17 November 1858 at 00:00 and 23 October 2026 at 00:00 UTC is MJD 61336.
Is the Julian Day the same as a Julian date code like 26296?
No. Codes like 26296 are a year plus a day of the year, used on packaging and in business software; see the Julian date converter. The astronomical Julian Day counts days since 4713 BC.
What is J2000?
J2000.0 is the standard reference epoch in astronomy: JD 2451545.0, which is 1 January 2000 at 12:00 Terrestrial Time, or 11:58:55.816 UTC.