Bissextile

Leap year every fourth year in which a day is added to the month of February on account of the excess of the tropical year 365 d 5 h 48 m 46 s above 365 days But one day added every four years is equivalent to six hours each year which is 11 m 14 s more than the excess of the real year Hence it is necessary to suppress the bissextile day at the end of every century which is not divisible by 400 while it is retained at the end of those which are divisible by 400