The Gregorian calendar was introduced by Pope Gregory XIII in 1582 and is now the world's most widely used civil calendar.
It was created to correct the gradual drift between the calendar date and the astronomical seasons that had accumulated under the Julian calendar. The reform adjusted the leap-year rule: century years are not leap years unless divisible by 400. Thus 2000 was a leap year, while 1900 was not.
The first countries adopted the reform in 1582, but other countries changed at different times. This staggered adoption explains why historical documents can show different dates for the same event. Britain and its colonies adopted the Gregorian calendar in 1752, while Russia adopted it for civil use after the 1917 Revolution.
The calendar is often called the “Western” calendar, but it is used internationally for civil and business purposes. Many communities continue to observe religious or cultural calendars alongside it.