Julian Day Converter

What Is a Julian Day Number?

A Julian Day Number (JDN) is a continuous count of days since the start of the Julian Period — January 1, 4713 BC in the proleptic Julian calendar. It was introduced by the French scholar Joseph Justus Scaliger in 1583 as a way to unify different historical calendar systems into a single, unambiguous timeline.

Unlike calendar dates, Julian Day Numbers never reset, never have leap year complications, and work seamlessly across the Julian-to-Gregorian calendar reform of 1582. This makes them the preferred format in astronomy, archaeology, and any field that needs to calculate the exact number of days between two historical dates.

How to Use the Julian Day Converter

1. 1The panel at the top shows today's Julian Day Number and Modified Julian Date.
2. 2To convert JDN to date: click "JDN → Date", enter the Julian Day Number, and click Convert.
3. 3To convert date to JDN: click "Date → JDN", pick a Gregorian date, and click Convert.
4. 4Results include the Julian Day Number, Julian Date (decimal), and Modified Julian Date.
5. 5The converter uses the standard proleptic Gregorian calendar algorithm.

Key Julian Day Number Examples

Julian Day vs. Modified Julian Date vs. Julian Calendar

Why Astronomers Use Julian Day Numbers

Astronomers need to calculate precise time intervals between observations that may be centuries apart. Calendar systems complicate this — the Gregorian reform skipped 10 days in 1582, leap year rules differ between Julian and Gregorian calendars, and historical records use inconsistent systems.

Julian Day Numbers solve all of this. To find the number of days between any two dates, simply subtract their JDNs. No calendar arithmetic, no leap year edge cases, no calendar reform gaps. This is why every major astronomy software, ephemeris, and space agency uses JDN as its internal time reference.

Julian Day Number Formula

For a Gregorian date (Y, M, D), the Julian Day Number is calculated as: