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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
`http://maia.usno.navy.mil/iauc19/iaures.html#b1`