The Julian Predecessor

Before the calendar we use today, the Roman Empire and much of the Western world used the Julian calendar, introduced by Julius Caesar in 45 BC. The Julian calendar assumed a year was exactly 365.25 days long and added a leap day every four years without exception.

The Drift

Because the actual solar year is 365.24219 days, the Julian calendar overestimated the length of the year by about 11 minutes. Over centuries, these 11 minutes added up. By the 16th century, the calendar had drifted by 10 days, causing the spring equinox to fall on March 11 instead of March 21. This was a major problem for the Catholic Church, as it affected the calculation of Easter.

The Gregorian Reform

In 1582, Pope Gregory XIII introduced the Gregorian calendar. To fix the drift, he ordered that 10 days be skipped; Thursday, October 4, 1582, was followed directly by Friday, October 15, 1582.

More importantly, he refined the leap year rule: century years (like 1700, 1800, 1900) would no longer be leap years unless they were divisible by 400 (like 1600 and 2000). This tiny adjustment brought the calendar year incredibly close to the actual solar year.

Modern Date Calculations

Today, almost all global commerce, technology, and date calculation tools—including our Days Between Dates Calculator—rely on the precise mathematical rules established by the Gregorian calendar over 400 years ago.