Convert between the Julian and Gregorian calendars
The Julian calendar was used across Europe until the Gregorian reform of 1582, which corrected an accumulating drift by removing ten days and tightening the leap-year rule. As a result, historical dates recorded in the Julian system sit several days apart from the modern Gregorian calendar — a gap that grows over the centuries.
How it works
The converter routes both calendars through the Julian Day Number (JDN), a continuous count of days that is independent of any calendar:
- The input date is converted to its JDN using that calendar’s leap-year rule.
- The JDN is converted back into the other calendar’s year, month and day.
Because every conversion passes through the same day count, the two directions are exact inverses, and the JDN is shown so you can verify the offset yourself. The difference between the calendars depends on the century:
| Gregorian period | Julian is behind by |
|---|---|
| 1500–1699 | 10 days |
| 1700–1799 | 11 days |
| 1800–1899 | 12 days |
| 1900–2099 | 13 days |
Example
The Gregorian date 14 March 1879 corresponds to the Julian date 2 March 1879 — a 12-day gap for the 19th century. Convert the other way and 2 March 1879 (Julian) returns 14 March 1879 (Gregorian), with the same Julian Day Number on both sides.
Pick a direction, enter the year, month and day, and the equivalent date appears instantly — all in your browser, nothing uploaded.
Why the Julian calendar drifts
The Julian calendar assumes every fourth year is a leap year, adding one extra day every 1,461 days. The actual solar year is approximately 365.2422 days — slightly less than 365.25 — so the Julian calendar gains roughly one day every 128 years against the seasons. By 1582, when Pope Gregory XIII ordered the reform, the calendar had slipped about ten days forward relative to the spring equinox, which mattered for the calculation of Easter.
The Gregorian reform corrected this in two ways. First, it dropped ten days immediately (in October 1582, October 4 was followed by October 15 in Catholic countries that adopted it). Second, it refined the leap-year rule: century years are now only leap years when divisible by 400, so 1700, 1800, and 1900 were not leap years under the Gregorian calendar but would have been under the Julian. That is why the gap between the two calendars increases by one day at each non-400 century year.
When you need this conversion
Historical research is the most common use. Primary sources from England before 1752, Russia before 1918, and the Eastern Orthodox Church (which still uses the Julian calendar for its liturgical year) record dates in the Julian system. Converting them to Gregorian lets you align them with other sources or with a modern calendar.
Astronomical records are another case. Historical observations of eclipses, comets, and planetary positions are often recorded in Julian dates so that they can be compared across calendar systems using the Julian Day Number as a neutral common reference.
Leap years compared
| Calendar | Leap year rule |
|---|---|
| Julian | Every year divisible by 4 |
| Gregorian | Divisible by 4, except century years unless divisible by 400 |
Years 100, 200, 300, 500 … are leap years in the Julian calendar but not the Gregorian (unless divisible by 400). Each such mismatch is where the calendars diverge by one more day.
Notes on pre-1582 dates
Before the Gregorian reform, any date in the Julian calendar can be expressed as a proleptic Gregorian date by applying the Gregorian rules backward. This tool supports such conversions: enter any year including years BC (which historians write as negative years or year 0) and the Julian Day Number arithmetic handles them correctly.