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From Citizendium, the Citizens' Compendium
Calendrics is the study of the algorithms for the generation of calendars.
A moment is a sufficiently small piece of time.
A day is a contiguous set of moments lasting approximately twentyfour hours. The precise length of a day depends on how the day is defined in the context of a particular calendar.
A date is the name associated with a given day according to some calendar.
Finally, a calendar is an algorithm for systematically assigning names to dates