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The Newton Calendar was devised by Sir Isaac Newton around 1700, when the Julian calendar was still in force in England and about to be set off another day from the Gregorian calendar. His papers include several slightly different drafts.

It features a five millenia leap cycle, where 29 February is dropped in centennial years (as in the Gregorian calendar), but not in those divisible by 500; however, an exceptional 30 February would be added in any year divisible by 5000:

365 + 1/4 - 1/100 + 1/500 + 1/5000 = 365.2422 = 365 d 5 h 48 min 46.08 s

Optionally, all winter months would be changed to have 30 days and all summer months had 31 and the leap day would be the last day of the last month of summer (September).

Its great lunar cycle of 49 months or 1447 days is subdivided into three sub-cycles of 17, 15 and 17 months, respectively, wherein all 26 odd months would have 30 days, the 23 even ones had 29 days.

1447 / 49 = ca. 29.530612245 = 29 d 12 h 44 min 4.9 s

1009 great cycles with an additional 32 months and another day would make 4000 years – 2 leap months (59 days) would be added every 250 years.

Easter would be on the Sunday after the 14th day of the lunar month that began after 7 March. Christmas could be moved to the winter solstice and Lady Day to the spring equinox.

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