Julian Date Converter
The Julian Date is the continuous day count astronomers use to avoid the irregular lengths of calendar months and years: it counts days, and fractions of a day, since noon Universal Time on 1 January 4713 BC. Because every instant maps to a single decimal number, the interval between any two events becomes a simple subtraction. This converter takes a Gregorian date and a time of day in Universal Time and returns the full Julian Date, the integer Julian Day Number, and the Modified Julian Date, using the standard Fliegel and Van Flandern algorithm so the output matches the U.S. Naval Observatory.
Julian Date formula
a = floor((14 - month) / 12)
y = year + 4800 - a, m = month + 12a - 3
JDN = day + floor((153m + 2)/5) + 365y + floor(y/4) - floor(y/100) + floor(y/400) - 32045
JD = JDN + (hour - 12)/24 + minute/1440
MJD = JD - 2400000.5
The integer JDN gives the Julian day starting at noon. Adding the time of day, offset by 12 hours because the Julian day begins at noon, yields the full Julian Date. The Modified Julian Date subtracts a fixed constant to start at midnight.
Julian Date notes
- The epoch is noon Universal Time on 1 January 4713 BC (Julian proleptic).
- A Julian Date ending in .5 corresponds to midnight; .0 corresponds to noon.
- The Modified Julian Date equals the Julian Date minus 2,400,000.5.
- Times here are treated as Universal Time, not local time.
- The formula is valid for Gregorian calendar dates.
Julian Date: frequently asked questions
What is a Julian Date?
The Julian Date is a continuous count of days, and fractions of a day, since noon Universal Time on 1 January 4713 BC in the proleptic Julian calendar. Astronomers use it because it removes the irregular lengths of months and years, making the interval between any two events a simple subtraction.
What is the difference between Julian Date and Julian Day Number?
The Julian Day Number is the integer count of whole days, defined to start at noon. The Julian Date adds the fraction of the day since the preceding noon. So a Julian Date of 2460000.5 means noon plus half a day, that is midnight, of the Julian Day Number 2460000.
Why does the Julian day start at noon?
It starts at noon Universal Time so that a single night of astronomical observation falls entirely within one Julian day rather than being split across a midnight boundary. This convention dates from the 19th century and remains the standard in astronomy.
What is the algorithm used here?
The calculator uses the standard Fliegel and Van Flandern integer formula for the Julian Day Number of a Gregorian calendar date, then adds the time of day as a fraction with the noon offset. The result matches the U.S. Naval Observatory Julian Date converter for Gregorian dates.
What is the Modified Julian Date?
The Modified Julian Date, or MJD, is the Julian Date minus 2,400,000.5. It starts at midnight rather than noon and uses smaller numbers, which is convenient for modern computing and satellite work. The calculator reports the MJD alongside the full Julian Date.
Official sources
- U.S. Naval Observatory: Julian Date Converter.
- U.S. Naval Observatory: Astronomical Applications.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.