GREGORIAN CALENDAR – (New Calendar) The Gregorian calendar is the calendar in contemporary use in most countries. During the Middle Ages, some problems were discovered with the use of the Julian Calendar: every century had three to four days too many. In the sixteenth century the mistake grew to 10 days. Therefore, in 1582, it was decided that the calendar needed reform. Pope Gregory XIII decreed that October 4 should be followed by October 15 at once. Also, he decided that all of the leap days of the full century years which were not dividable by 400 would be omitted. In this manner, 1900 was not a leap year, 2000 was a leap year, and 2100 will not be. The average duration of the Gregorian year is 365.2425 days.