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In page Intercalation (timekeeping):

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The tabular Islamic calendar, used to predict the new moon and for historical date conversions, has 12 lunar months that usually alternate between 30 and 29 days every year, but the algorithm adds an intercalary day as the last month of the year 12 times in a 33-year cycle.[citation needed] Some historians also linked the pre-Islamic practice of Nasi' to intercalation.