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Assuming a 90 year spell includes 22 leap years, the number of days is equal to (68 x 365) + (22 x 366) = 24820 + 8052 = 32872 days. If the 90 year spell includes 23 leap years, the number of days is equal to (67 x 365) + (23 x 366) = 24455 + 8418 = 32873 days. 3 people found this useful.
That would be 17 total days off a year. (260.9-17)/12 is approximately 20 5/16 days. Which, as I said before, is an average and only truly accurate over a period of 28 years and therefore could be very different from the actual number.