The most widely used calendar internationally is the Gregorian calendar which, as Wikipedia will tell you, is also called the Western calendar or Christian calendar. It's essentially based on the approximately 365-day solar cycle, divided into the twelve months which are related to the lunar cycle. The name "Gregorian" is due to the major reforms enacted by Pope Gregory in the 16th century to correct problems arising from inaccuracies in the old Roman Julian calendar then in use.
Gregorian calendar - Wikipedia, the free encyclopedia
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