Julian vs Gregorian Calendar: The Difference Explained
Compare the Julian and Gregorian calendars, their leap-year rules, and why different historical dates can appear ten days apart.
The Julian and Gregorian calendars both use a 365-day year with occasional leap days, but their different leap-year rules gradually place the same named date on different solar days.
The Julian Rule
Julius Caesar's calendar added a leap day every four years. That creates an average year of 365.25 days. It is close to the seasonal year, but slightly too long, so the calendar slowly drifts relative to equinoxes and seasons.
The Gregorian Adjustment
The Gregorian calendar keeps the every-fourth-year rule, except century years are not leap years unless divisible by 400. Thus 1600 and 2000 are leap years, while 1700, 1800, and 1900 are not. Its average year, 365.2425 days, is much closer to the solar year.
Why Ten Dates Vanished in 1582
By 1582, the accumulated Julian drift was ten days. Catholic countries adopting Pope Gregory XIII's reform moved from October 4 directly to October 15. The next day was not physically lost; only the civil date label changed.
Countries adopted the reform at different times. This is why a historical event can be recorded with two dates, marked Old Style and New Style. A date conversion must identify the place and the calendar in use, not simply subtract a fixed number of days.
The Gap Changes Over Time
The difference was 10 days in 1582, 11 days after 1700, 12 after 1800, and 13 after 1900. It will grow again after 2100 because the Gregorian calendar will skip that Julian leap day.
For modern international records, use the Gregorian calendar and an unambiguous numeric date such as 2026-07-18. For history, preserve the source calendar alongside any converted date.