Julian Date Converter
Convert between a Gregorian calendar date and its Julian Day Number in both directions, using the standard astronomical day-count algorithm.
Calculator by Toolsloft ↗- Julian Day Number
- 2451545
Convert a calendar date to its Julian Day Number and back. The Julian Day Number is a running count of days used in astronomy and software to compare dates without months or years getting in the way. Enter a date to get its Julian Day, or a Julian Day to get the calendar date.
How this is calculated
The conversion uses the standard integer algorithms for the proleptic Gregorian calendar published by Fliegel and Van Flandern. Each calendar date maps to one whole Julian Day Number, and the inverse algorithm recovers the date exactly, so the two directions are consistent round trips.
How to use
- Choose whether to convert a date to a Julian Day Number or the other way.
- Enter the date in YYYY-MM-DD format, or enter a whole Julian Day Number.
- Read the converted value from the result.
Examples
- Y2K epoch:
2000-01-01 = Julian Day 2451545 - Unix epoch:
1970-01-01 = Julian Day 2440588 - Reverse:
Julian Day 2451545 = 2000-01-01
FAQ
- What is a Julian Day Number?
- It is a continuous count of days used mainly in astronomy, starting from a reference point in 4713 BC. Because it is a single whole number, the gap between any two dates is just their difference in Julian Day Numbers.
- Is this the same as the Julian calendar?
- No. The Julian Day Number is a day count and is unrelated to the old Julian calendar. This tool uses the proleptic Gregorian calendar for the date side of the conversion.