Why the Calendar Needed Reform

Before the Gregorian calendar, much of the world used the Julian calendar. Introduced by Julius Caesar in 45 BCE, the Julian calendar had a leap year every four years. However, it slightly overestimated the length of a year by about 11 minutes.

Over centuries, this small error added up, causing calendar dates to drift relative to the equinoxes.

Enter Pope Gregory XIII

To fix this drift, Pope Gregory XIII introduced the Gregorian calendar in 1582. His reform:

• Skipped 10 days to realign the calendar with the equinox.
• Introduced a new leap year rule: years divisible by 100 are not leap years unless divisible by 400.

Adoption Around the World

Catholic countries adopted the Gregorian calendar quickly, but others delayed. For example, Great Britain and its colonies switched in 1752—by then, the calendar was 11 days off.

Some countries didn't switch until the 20th century. Today, it's the most widely used calendar in the world.

Fun Fact

When the calendar changed in Britain in 1752, people went to bed on September 2 and woke up on September 14!

Conclusion

The Gregorian calendar was a necessary correction that keeps our civil and religious events aligned with the solar year. It’s a great example of how science and society shape timekeeping.

Explore more about calendars and time on our Time Blog.