Julian Date Calculator
Convert any Gregorian calendar date to its Julian Day Number (and back), and see the day-of-year ordinal date alongside it.
Calendar date → Julian Day Number
Julian Day Number → calendar date
Frequently Asked Questions
What is a Julian Day Number?
A single integer that counts days continuously since January 1, 4713 BCE (proleptic Julian calendar, noon UTC), used in astronomy and some data systems because it never has to account for months, years or leap days — it's just a number that goes up by one every day. January 1, 2000 is JDN 2,451,545, a widely published anchor value.
Is a Julian Day Number the same as the "Julian calendar"?
No, and this is the most common mix-up with the term. The Julian calendar was the calendar system Europe used before switching to the Gregorian calendar (in 1582, later in some countries), with a slightly different leap-year rule. A Julian Day Number is a continuous day count that works alongside either calendar system — this page converts between it and the modern Gregorian calendar, not the historical Julian calendar.
What's the day-of-year number?
Also called the ordinal date: January 1 is day 1, December 31 is day 365 (or 366 in a leap year). Some file-naming conventions, military logistics, and older data formats use "YYDDD" style dates built on this number instead of month and day.
How is the Julian Day Number actually calculated here?
By converting the calendar date to a day count relative to a fixed epoch, then adding the published offset between that epoch and day 0 of the Julian Day system. The algorithm (Howard Hinnant's days_from_civil) is the same one C++20's <chrono> library uses, and is correct for the entire proleptic Gregorian calendar.