๐ Julian Day Converter
Converts between a Gregorian calendar date and the Julian Day Number (JDN) โ a continuous day count starting from noon UTC on January 1, 4713 BCE, widely used in astronomy. The Modified Julian Day (MJD) is shown as well.
How to use
- Enter a date to automatically calculate its Julian Day Number (JDN).
- Or enter a JDN to automatically calculate the corresponding calendar date.
- The Modified Julian Day (MJD = JD - 2400000.5) is shown alongside the result.
How the calculation works
The Julian Day Number (JDN) numbers days continuously from noon Universal Time on January 1, 4713 BCE (day 0). Because it ignores the varying lengths of months and years, the number of days between two dates is a simple subtraction, which is why astronomy and calendar calculations use it. This tool converts between Gregorian dates and Julian Day Numbers and also shows the Modified Julian Day (MJD). A Julian Date (JD) starts each day at noon, so midnight UT of a date is JD = JDN โ 0.5. The Modified Julian Day is defined as MJD = JD โ 2400000.5, which is a whole number at midnight UT (MJD = JDN โ 2400001).
Worked example
January 1, 2000 โ JDN 2451545 (a common reference date in astronomy) January 1, 1970 โ JDN 2440588 (the start of Unix time) September 19, 2026 โ JDN 2461303, MJD 61302 November 17, 1858 โ MJD 0 (the start of the Modified Julian Day) Days between two dates: 2461303 โ 2451545 = 9758 days
Things to be aware of
- Dates before October 15, 1582 are calculated in the proleptic Gregorian calendar (the Gregorian rules extended backwards). They differ from Julian calendar dates found in historical sources.
- The name is usually said to come from Julius Scaliger, the father of its inventor, rather than from the Julian calendar of Julius Caesar.
- Some browsersโ date pickers cannot enter BCE (negative) years. Enter those through the Julian Day Number field instead.
FAQ
What is a Julian Day Number (JDN)?
A date representation that counts days continuously as an integer, starting from noon (UTC) on January 1, 4713 BCE. Itโs used in astronomy and calendar calculations as a common reference point for comparing different calendar systems.
What is the Modified Julian Day (MJD)?
The Julian Date (JD) minus 2400000.5, which makes midnight UT on November 17, 1858 equal to 0. It is a whole number at midnight, and this tool shows that midnight value for the date (JDN โ 2400001). It is widely used in modern science, such as satellite orbit calculations.
Does this use the Julian or Gregorian calendar?
This tool calculates based on the Gregorian calendar in current use. Dates before October 15, 1582 are treated using the โproleptic Gregorian calendarโ โ the Gregorian calendar extended backward in time.