Taking Months to Bits
A calendar system is an attempt to make sense of the passing of time with respect to astronomical phenomena — the positioning of the Sun, the Moon and the stars with respect to our unfixed and moveable Earth. It is perhaps ironic, then, that the widely used Gregorian calendar system is a cobbled-together mishmash that better reflects the arbitrariness of history than it does either celestial order or logical consistency.
Source: Taking Months to Bits. A calendrical journey into low-level programming, an article by Kevlin Henney.