http://astro.nmsu.edu/~lhuber/leaphist.html A detailed essay about calendars and their history, reprinted from the Explanatory Supplement to the ...
A calendar is a physical device (often paper) (for example, a desktop calendar or wall almanac); one sheet can show a single day, a week, a month, or a year. It can also be in electronic format, for example online calendars allowing you to share calendar events, etc. with other web users.
A calendar is a system for naming periods of time, typically days. These names are known as calendar dates. Cycles in a calendar are often synchronised with the perceived motion of astronomical objects.
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