Bissextile

[Sarah]

Leap year; every fourth year, in which a day is added to themonth of February on account of the excess of the tropical year (365d. 5 h. 48 m. 46 s.) above 365 days. But one day added every fouryears is equivalent to six hours each year, which is 11 m. 14 s. morethan the excess of the real year. Hence, it is necessary to suppressthe bissextile day at the end of every century which is not divisibleby 400, while it is retained at the end of those which are divisibleby 400.

[Rihanna]

(of a year) having an extra day on 29 February: