What is a Julian Day Number?

The Julian Day Number is a continuous count of days used in astronomy and calendar calculations. JDN 0 corresponds to 1 January 4713 BC in the proleptic Julian calendar, and the count increases by one each day.

How is JDN different from a Julian date?

A Julian Day Number is an integer for a whole day at noon. A full Julian date adds a fractional part for the time of day. This tool works with the integer day number, which is what most date arithmetic needs.

Which calendar does this tool use?

It uses the proleptic Gregorian calendar, meaning the Gregorian rules are applied to all dates including those before the 1582 reform. This is the convention used by ISO-8601 and most software.

Can it handle BC dates?

Yes. Enter an astronomical year number, where the year 0 is 1 BC and the year -1 is 2 BC. Negative years are accepted in the date field for ancient and astronomical work.

Why is the weekday shown?

Because JDN 0 fell on a Monday, the day of the week follows directly from the number modulo 7. Showing it gives a fast independent check that the conversion landed on the date you expected.

Julian Day Number Converter

What the Julian Day Number is for

The Julian Day Number, or JDN, is a single integer that counts whole days from a fixed origin far in the past. Because it is just a count, the number of days between two dates is a simple subtraction, which makes the JDN the backbone of astronomical timekeeping and many calendar routines. This tool converts a Gregorian date to its JDN and converts a JDN back to a date.

How it works

The conversion uses the algorithm published by Fliegel and Van Flandern in 1968, which computes the JDN with integer arithmetic and no lookup tables. Going from a date, it shifts the year and month so that the leap-day boundary falls at the end of the cycle, then sums the contributions of years, the leap-year corrections for the Gregorian rule, and the day of the month.

2000-01-01  ->  JDN 2451545
2024-02-29  ->  JDN 2460370

The reverse direction inverts the same arithmetic to recover the year, month, and day from the integer. After converting from a date, the tool also checks that the JDN maps back to the exact day entered, which rejects impossible dates such as the 30th of February.

Example and notes

The reference date is 2000-01-01, JDN 2451545, a useful anchor to remember. The weekday is derived directly from the number: since JDN 0 was a Monday, you can compute the day of the week from the JDN modulo 7 without any further calendar logic. All dates are interpreted in the proleptic Gregorian calendar, so results before the 1582 reform follow the Gregorian rules rather than the historical Julian calendar.