365

+ Neither 7 (the number of days in a week) nor 31 (the most frequent number of days in a month) are prime factors of 365 (the most frequent number of days in a year). Instead, its prime factors are 5 and 73, numbers which don't appear as part of any cycle in the Gregorian calendar. [Raffill]

(There are 2 curios for this number that have not yet been approved by an editor.)

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