Pope Gregory XIII (1502-1585). Statue of the Italian educator Ugo Buoncompagni, who was pope of the Roman Catholic Church between 1572-1585. He was instrumental in the revision of the Julian calendar, which had be introduced by Julius Caesar in 45BC. This new Gregorian calendar had been devised to closely approximate the tropical year, which is the actual length of time it takes for the Earth to complete an orbit of the Sun (365.242190 days). Many European Catholic countries adopted the new calendar when it was introduced in 1582, which required that 10 days be lost from the first year, and that every fourth year (leap) there is an extra day in February.