According to the
Gregorian calendar, the 'calendar year' begins on
January 1 and ends on
December 31. Other alignments of the 12-month period can be used for purposes of
accounting (see
fiscal year).
Generally speaking, a calendar year begins on the
New Year's day of the given calendar system and ends on the day before the following New Year's day.
By convention, a calendar year consists of a
natural number of
days. To reconcile the calendar year with an astronomical cycle (which could not possibly be reckoned in a whole number of days), certain years contain
extra days. Through further insertion of non-day units of time, the calendar year can be further synchronized; however, these extra units of time are not considered part of the calendar.
See also
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Calendar
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Calendar reform
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Fiscal year
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ISO 8601
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ISO week date
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Leap year
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Seasonal year
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Year