On 24 February 1582, Pope Gregory XIII decreed the introduction of the Gregorian calendar that was named after him. The previously valid Julian calendar was based on a year duration of 365.25 days. Compared to the solar year, the Julian year was 11 minutes and 14 seconds too long, which, in the course of several centuries, produced a growing deviation from the actual progression of the sun. In the 16th century, the difference had already grown to ten days.
Today, the Gregorian calendar is the most commonly used calendar in the world. It attempts to map the precise astronomical duration of a year (a solar year) of 365.24219 days and has the following rules:
- Years divisible by four are extended with a leap day, giving the year an average of 365.25 days.
- The leap day is omitted in years that are divisible by 100. The year has an average of 365.24 days.
- Finally, the leap day is again added to years divisible by 400. Now, a year has an average duration of 365.2425 days, coming very close to the solar year.