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.