 The accumulated effect of this difference over some 1600 years since the basis for calculation of the date of Easter was determined at the First Council of Nicea means for example that, from 29 February "Julian" (13 March "Gregorian") 1900 and until 28 February "Julian" (13 March "Gregorian") 2100, the "Julian" calendar is 13 days behind the "Gregorian" calendar; one day after (i.e.on 29 February "Julian" or 14 March "Gregorian"), the difference will be 14 days.Although the new calendar was much simpler than the pre-Julian calendar, the pontifices initially added a leap day every three years, instead of every four.