Julian Day Calculator – Convert Dates to Julian Day Numbers
The Julian Day Calculator converts any calendar date and time into its Julian Day Number (JDN) and fractional Julian Date (JD) — a continuous day count used by astronomers, historians, and software engineers to do date arithmetic without worrying about months, leap years, or calendar reforms. It also converts a Julian Day value back into a human-readable calendar date, computes the Modified Julian Date (MJD), and finds the number of days between two dates.
What Is a Julian Day Number?
The Julian Day count began at noon UTC on January 1, 4713 BCE (the proleptic Julian calendar epoch) and has increased by exactly one every day since. Because it is a single running number rather than a year/month/day triplet, it sidesteps the need for calendar-specific logic — subtracting two Julian Day values instantly gives the number of days between them, regardless of which months or years they fall in. This makes it indispensable in astronomy for computing the position of celestial bodies and in software for reliable date arithmetic.
JD, JDN, and MJD Explained
The JDN is the whole-number count of days. The full Julian Date (JD) adds a decimal fraction representing the time of day, using the formula JD = JDN + (hour − 12) / 24 + minute / 1440 + second / 86400. Because the Julian Day count increases at noon UTC rather than midnight, a JD ending in .0 always falls at noon, while .5 falls at midnight. The Modified Julian Date is simply MJD = JD − 2400000.5, which shifts the reference point to midnight and produces smaller, easier-to-read numbers — this is the convention used by GPS, satellite tracking, and many scientific data formats.
Julian vs. Gregorian Calendar Dates
Most of the world uses the Gregorian calendar, adopted in October 1582, which refines the leap-year rule so century years are only leap years when divisible by 400. Older records, and some Orthodox Christian traditions, still use the Julian calendar, whose simpler rule treats every year divisible by 4 as a leap year. Selecting the correct calendar system for your input date matters most for dates before 1582 — this calculator lets you choose either system and converts between them using the Julian Day Number as a common reference point.
Reverse Conversion and Day Differences
Switching the conversion direction lets you enter a JD or MJD value and recover the exact calendar date and UTC time it represents, in either the Gregorian or Julian calendar. The Day Difference tab converts two calendar dates to their Julian Day Numbers and subtracts them, giving an exact day count that automatically accounts for leap years without any manual calendar math.
Who Uses Julian Day Calculations
Astronomers use Julian Dates to time celestial events and satellite passes precisely. Historians and genealogists use them to compare dates recorded under different calendar systems across centuries. Software developers use the JDN as a robust internal representation for date arithmetic, and researchers use MJD when working with scientific datasets that adopt that convention by default.
Accuracy and Supported Range
This calculator supports proleptic calendar dates from astronomical year -4712 (4713 BCE) through 9999CE, using the well-established Fliegel & Van Flandern algorithm for forward conversion and its standard inverse for reverse conversion. Results are exact for any valid date within this range on both the Gregorian and Julian calendars.