Julian day number calculator
Astronomers and programmers use the Julian Day Number to count days without months or years; this tool gives four common day counts at once.
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How to use it
- Enter date.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
JDN = days since 1970-01-01 + 2440588, the number of 1970-01-01. JD at 00:00 UT = JDN - 0.5. MJD = JD - 2400000.5. Rata Die = days since 1970-01-01 + 719163, where 0001-01-01 = 1.
Example
Inputs: Date = 2000-01-01. Result: Julian Day Number (JDN): 2,451,545; Julian Date at 00:00 UT: 2,451,544.5; Modified Julian Date (MJD): 51,544.
Limits and caveats
- The input is a Gregorian date even before 1582; it does not accept old Julian calendar dates.
- The JDN day begins at noon UT, so the JD at midnight is half a day lower.
Questions about this tool
What is the difference between JDN and JD?
JDN is a whole number for the day, while JD adds the fraction of the day. The Julian day starts at noon, so midnight is JDN - 0.5.
Is the Julian Day Number related to the Julian calendar?
No, despite the name. The Julian Day Number is a continuous day count that starts in 4713 BC.
Where is MJD used?
In astronomy, satellite work and timekeeping, because it is smaller and starts at midnight on 17 November 1858.