is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) compared to a common year.The length of a day is also occasionally corrected by inserting a leap second into Coordinated Universal Time (UTC) because of variations in Earth's rotation period.The 366th day (or 13th month) is added to keep the calendar year synchronized with the astronomical year or seasonal year.[1] Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons.Leap years can present a problem in computing, known as the leap year bug, when a year is not correctly identified as a leap year or when 29 February is not handled correctly in logic that accepts or manipulates dates.Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable.