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Julian Day Numbers were introduced by astronomers in the 19th century C.E. as a continuous day numbering scheme without years or leap days. The epoch (the start of day 0) is 1 January -4712 C.E. at 12:00:00 TT (by recommendation of the International Astronomical Union). For precise astronomical calculation, fractional Julian day numbers are used. The calendar function returns fractional Julian day numbers, assuming that the fractional part of the day specification in the source calendar is measured since the midnight before the noon corresponding to that date. This means that an integer date in the source calendar corresponds to a JD (= Julian Date) ending in ".5". To get the integer count, round to the nearest integer; round JDs ending in ".5" up to the next greater integer.

For the official definition of the Julian Date by the International Astronomical Union, see at