Julian Day (JD) to date converter
The Julian Day is a continuous day count used in astronomy, and each JD day starts at noon UTC.
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How to use it
- Enter julian day (jd) and time zone (optional).
- 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
Date = 1970-01-01 00:00 UTC + (JD - 2440587.5) days. JDN = floor(JD + 0.5). MJD = JD - 2400000.5.
Example
Inputs: Julian Day (JD) = 2,451,545, Time zone (optional) = UTC. Result: Date and time (UTC): 2000-01-01 12:00:00; Weekday: Saturday.
Limits and caveats
- The main date uses the proleptic Gregorian calendar even before 1582, which is why the Julian calendar date is shown too.
- Supported range is years 1 to 9999, with millisecond precision.
Questions about this tool
Why does the Julian Day start at noon?
Astronomers chose noon so the date would not change in the middle of a night of observing. So JD 2451545.0 is noon on 1 January 2000.
What is the difference between JD and JDN?
JD is a decimal that includes the time of day; JDN is the whole-day integer, equal to floor(JD + 0.5).
Is the Julian Day related to the Julian calendar?
Not directly. It is a separate day count that starts in 4713 BC, while the Julian calendar is the calendar used before the Gregorian one.