📜 Julian Calendar & Julian Day Converter
Julian ↔ Gregorian for historical dates, plus the astronomical Julian Day Number — two different "Julian" dates, both handled precisely and locally.
Julian
5 July 2026 (Julian)
Saturday · The pre-1582 Western calendar; now ~13 days behind Gregorian.
Julian → Gregorian
Sunday, January 10, 1447
Gregorian · 1447-01-10 · resolves Julian: 1 January 1447 (Julian)
Forward conversion uses your browser's built-in ICU calendar data; reverse inverts it exactly. All computed on your device.
How the julian calendar & julian day converter works
Two things share the name "Julian". The Julian calendar is the Roman calendar used across the West until 1582, with a simple leap year every four years; it has drifted about 13 days behind the Gregorian calendar. The Julian Day Number is unrelated — a continuous count of days used in astronomy since 4713 BC. This tool converts Gregorian to and from the Julian calendar using the standard integer (Fliegel) algorithm, and also shows the Julian Day Number, both computed exactly on your device.
Watch the 1582 changeover: when the Gregorian calendar was introduced, Thursday 4 October 1582 (Julian) was followed directly by Friday 15 October 1582 (Gregorian) — ten days were skipped. Different countries adopted the reform in different years (Britain and its colonies not until 1752, skipping 11 days), so a "date" in an old document depends on which calendar the writer used.
Frequently asked questions
What is the difference between the Julian calendar and Julian Day Number?
The Julian calendar is a calendar system (months and years, leap year every 4 years) used before the Gregorian reform. The Julian Day Number is a continuous count of days used in astronomy, with no months. They share a name but are otherwise unrelated; this tool handles both.
How many days is the Julian calendar behind the Gregorian?
Currently 13 days, because the Julian calendar has a leap year every four years with no century exception, so it gains about three days every four centuries. The gap grows by a day roughly each century.
What happened in October 1582?
The Gregorian reform skipped ten days: 4 October 1582 (Julian) was followed by 15 October 1582 (Gregorian). Catholic countries adopted it then; others later — Britain in 1752, skipping eleven days.
What is a Julian Day Number used for?
Astronomers use it to give every day a single increasing number, so intervals between events in any calendar are a simple subtraction. It starts from 1 January 4713 BC and changes at noon universal time.
Is this accurate for ancient dates?
Yes — it uses exact integer arithmetic that works for any year, including proleptic (extrapolated) Julian dates before the calendar was introduced. Everything is computed locally.