The Gregorian calendar was introduced by pope Gregory XII in 1582 to replace the Julian calendar. The Gregorian calendar only differs from the Julian calendar in the distribution of leap years and by an initial shift of ten days with Friday 15 October 1582 (Gregorian) following Thursday 4 October 1582 (Julian). Leap years are the same as in the Julian calendar except for millennium years that are a multiple of 100 but not of 400. Thus 1700, 1800 and 1900 were common years, whereas 2000 was a leap year similarly to the Julian calendar. The average duration of the Gregorian year (365.2425 days) is a good approximation of the tropical year. This calendar is currently in use in most countries.