Description

This article is from the Calendars FAQ, by Claus Tondering claus@tondering.dk with numerous contributions by others.

2.2.2. Isn't there a 4000-year rule? (Gregorian calendar)

It has been suggested (by the astronomer John Herschel (1792-1871)
among others) that a better approximation to the length of the
tropical year would be 365 969/4000 days = 365.24225 days. This would
dictate 969 leap years every 4000 years, rather than the 970 leap
years mandated by the Gregorian calendar. This could be achieved by
dropping one leap year from the Gregorian calendar every 4000 years,
which would make years divisible by 4000 non-leap years.

This rule has, however, not been officially adopted.

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