History of the Gregorian Calendar
A concise overview of how the current civil calendar (the Gregorian calendar) came to be.
Background — the Julian calendar
The story begins with the Julian calendar, introduced by Julius Caesar in 45 BCE. It established a 365-day year with an extra day every 4 years (a leap year). That rule averages the year length to 365.25 days.
The actual tropical year is about 365.2422 days — roughly 11 minutes shorter than the Julian year. The small difference accumulated over centuries, slowly shifting the calendar away from astronomical events like the vernal equinox.
Why reform was needed
By the 16th century the date of the vernal equinox had drifted about 10 days earlier than at the time of the Council of Nicaea (325 CE). This created problems for calculating Easter and other observances.
The Gregorian reform (1582)
1582 — Pope Gregory XIII
- 10 days were skipped: Oct 4, 1582, was followed by Oct 15.
- Leap years remain every 4 years, except centuries not divisible by 400.
Adoption worldwide
Catholic countries switched in 1582. Britain and colonies adopted in 1752, Russia in 1918, Greece in 1923. Adoption was gradual worldwide.
Leap year rule
// A year is a leap year if:
if (year % 4 === 0) {
if (year % 100 === 0) {
return (year % 400 === 0);
}
return true;
}
return false;
This gives an average year length of 365.2425 days — close to the tropical year.
Legacy
The Gregorian calendar fixed the Julian drift and became the standard civil calendar worldwide. Its accuracy (1 day in ~3,300 years) ensures long-term seasonal alignment.
