Century leap year

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In the Gregorian calendar, an end-of-century leap year (often referred to as a century leap year) is a year that is exactly divisible by 400 and, as with every other leap year, qualifies for the intercalation of February 29. End-of-century years that are exactly divisible by 4 but not by 400 are common years. The years 1600, 2000 and 2400, for example, are end-of-century leap years. The end-of-century years 1700, 1800, 1900, 2100, 2200, and 2300 are non-leap years. End-of-century leap years always start on a Saturday, and thus the resulting upcoming February 29 is always on a Tuesday.

The end-of-century year "divisible by 400" rule of the Gregorian calendar was considered an improvement over the previously utilized Julian calendar which had provided for a leap year at four year intervals. Over time, the Julian practice resulted in too many leap days being added to the calendar, thus causing it to gradually drift with respect to the astronomical seasons of the years (and natural events, such as the spring equinox, to occur earlier and earlier in the calendar).

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  • Spofford, Thomas (1835). A new system of practical astronomy: made plain and easy to those who have not studied mathematics : containing the elementary principles of the science, all the rules and tables necessary for making all the calculations for an almanac ... Boston: Lemuel Gulliver. p. 28. 

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