|a calendar year of 366 days, February 29 (leap day) being the additional day, that occurs every four years (those whose number is divisible by four) except for century years whose number is not divisible by 400. It offsets the difference between the length of the solar year (365.2422 days) and the calendar year of 365 days|
year containing some intercalary period, especially a Gregorian year having a 29th day of February instead of the standard 28 days. The astronomical year, the time taken for the Earth to complete its orbit around the Sun, is about 365.242 days, or, to a first approximation, 365.25 days. To account for the odd quarter day, an extra calendar day is added every four years, as was first done in 46 BC, with the establishment of the Julian calendar. Over many centuries, the difference between the approximate value 0.25 day and the more accurate 0.242 day accumulates significantly. In the Gregorian calendar now in general use, the discrepancy is adjusted by adding the extra day to only those century years exactly divisible by 400 (e.g., 1600, 2000). For still more precise reckoning, every year evenly divisible by 4,000 (i.e., 16,000, 24,000, etc.) may be a common (not leap) year.
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