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A practical system for timetabling human affairs, taking into account important natural cycles such as the day, month and year. An ideal practical calendar needs to be organized so that the civil years keep step with the seasons.
Since the tropical year does not contain an integral number of days, devising such a calendar presented great difficulties for earlier civilizations.
The calendar now in general use throughout much of the world is the Gregorian calendar, introduced in 1582. The Gregorian calendar falls out of step with the seasons by only one day in 3,300 years. Other traditional calendars still in use include the Jewish, Muslim and Chinese calendars.
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