Julian Day Number Calculator
Convert a calendar date to its astronomical Julian day.
A single number for any date
The Julian Day Number counts days continuously from a fixed point far in the past, turning any date into a single integer. Astronomers use it so that the gap between two dates is just a subtraction.
The 1st of January 2000 is Julian Day 2,451,545. Enter any date and the tool returns its number, which is handy for astronomy, ephemerides, and date-difference code.
Precise and private
The calculation runs in your browser, so nothing you enter is uploaded.
Frequently Asked Questions
What is a Julian Day Number?
A continuous count of days since the start of the Julian Period in 4713 BC, used in astronomy to make date arithmetic simple.
Is it the Julian calendar?
No. Despite the name, the Julian Day Number is a day count, not the Julian calendar. This tool uses Gregorian dates.
Why is it useful?
Astronomers and programmers use it to compute intervals between dates without calendar complications.
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