The world runs on the Gregorian calendar — but it replaced an older one, the Julian calendar, only after a centuries-long drift forced a reform. The difference between them is small per year, but it adds up to a 13-day gap today. Here's what changed and why.
For the dedicated pages, see the Gregorian calendar and the Julian calendar.
The one difference that matters: leap years
Both calendars have 12 months and a leap day — but they handle leap years slightly differently:
| Julian calendar | Gregorian calendar | |
|---|---|---|
| Introduced | 45 BC (Julius Caesar) | 1582 (Pope Gregory XIII) |
| Leap year rule | Every 4 years, always | Every 4 years, except century years not divisible by 400 |
| Average year | 365.25 days | 365.2425 days |
| Drift vs the Sun | ~1 day every 128 years | ~1 day every 3,030 years |
The true solar year is about 365.2422 days. The Julian year (365.25) is ~11 minutes too long — tiny, but over centuries it pushed the calendar out of step with the seasons.
Why the reform happened
By the 1500s, the Julian drift had moved the spring equinox about 10 days away from its traditional date — a problem for the Church, which sets Easter from the equinox. In 1582, Pope Gregory XIII:
- Dropped 10 days — in Catholic countries, the day after 4 October 1582 was 15 October 1582.
- Refined the leap-year rule — century years (1700, 1800, 1900) are only leap years if divisible by 400 (so 2000 was, 1900 wasn't).
The 13-day gap today
Because the Julian calendar kept adding "extra" leap days the Gregorian skipped, the gap has grown from 10 days (1582) to 13 days now (and will reach 14 days in 2100). That's why churches still on the Julian calendar celebrate Christmas on 7 January — which is 25 December in the Julian reckoning.
Who adopted Gregorian when?
Adoption was gradual and uneven:
- 1582: Catholic countries (Spain, Portugal, Italy, Poland, France).
- 1752: Britain and its colonies (including America) — they dropped 11 days, sparking confusion.
- 1918: Russia, after the revolution (which is why the "October Revolution" actually happened in November by the Gregorian calendar).
- 1923: Greece, one of the last in Europe.
Who still uses the Julian calendar?
Several Orthodox churches (Russian, Serbian, Georgian, Jerusalem) still use it for fixed feast days — the reason for "Orthodox Christmas" on 7 January and "Old New Year" on 14 January.
Quick answers
What is the difference between the Julian and Gregorian calendars? The Gregorian calendar refined the leap-year rule (skipping 3 leap years every 400 years) to fix the Julian calendar's drift against the seasons.
Why is there a 13-day difference? Accumulated extra Julian leap days. The gap was 10 days in 1582 and is 13 days now.
When did we switch to the Gregorian calendar? 1582 in Catholic Europe; 1752 in Britain and America; later elsewhere (Russia 1918, Greece 1923).
Who still uses the Julian calendar? Some Orthodox churches, for liturgical dates — hence Orthodox Christmas on 7 January.