Date to Julian Day (JD) converter
Enter a date and time to get the Julian Day values used in astronomy and satellite data.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter date, time, seconds and time zone of the date entered.
- 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
JD = (Unix seconds / 86400) + 2440587.5. JDN = the UTC date day number = floor(JD + 0.5). MJD = JD - 2400000.5.
Example
Inputs: Date = 2000-01-01, Time = 12:00, Seconds = 0, Time zone of the date entered = UTC. Result: Julian Day (JD): 2451545.
Limits and caveats
- Time is treated as UTC after the zone conversion; it does not apply the UTC to Terrestrial Time (TT) difference some astronomy work needs.
- The date you enter is Gregorian, even before 1582.
Questions about this tool
What is the Julian Day at the start of 2000?
Midnight UTC on 1 January 2000 is JD 2451544.5, and noon that day is JD 2451545.0, the J2000 epoch.
Why is the JDN sometimes higher than the JD?
The JDN belongs to the whole date, while the JD only reaches it at noon. Before noon the JD is up to one day lower.
Where is MJD used?
In space science, satellite schedules and GPS timing, because it is shorter and its day starts at midnight.