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Julian date, also commonly called as Julian day, refers to the number of days passed since the beginning of a self-created cycle of 7980 years. This concept was introduced by Joseph Scaliger in 1538 and its importance is integral in number computation by determining the difference between two known dates. It is largely used in computers and astronomy.
Julian date is often confused with the Julian calendar, which was introduced by Julius Caesar in 45 B.C. to bring about reform in the Roman Empire. Julian date is the difference (or modulus) of time in days lapsed since the start of the 7,980-year cycle. This 7,980 years is calculated with several traditional time cycles (solar, lunar and a Roman tax cycle) for which 7,980 was a common multiple. The current Julian cycle started on January 1, 4713 B.C. (Gregorian calendar) and will end on January 22, 3268 A.D.
Julian date is commonly used in computer science to calculate the difference between days, since all numbers in the system are consecutive integers.