The Gregorian Calendar, developed as a way to correct the Julian Calendar
, is used by most people in the world today. Aloysius Lilius proposed the changes and Pope Gregory XIII (after whom the calendar is named Gregorian) proclaimed it by papal bull in 1582. The Julian Calendar was longer than the actual solar year, which meant the seasons were drifting, and so was the religious holiday of Easter. The Gregorian Calendar dropped 10 days and reduced the number of leap years, which, according to the Julian Calendar, were every 4 years. In the Gregorian Calendar, every year that can be divided by 4 is a leap year, but there are exceptions. Century years are excluded, except those century years that can be divided by 400: 2000 was a leap year; 2100 will not be.
While the Gregorian Calendar is used almost everywhere today, it took centuries for this to be true. Since the Gregorian Calendar was an innovation of the Catholic Church, its acceptance was resisted.