This function will return a Unix timestamp corresponding to the
Julian Day given in jday
or FALSE if
jday
is not inside the Unix epoch
(Gregorian years between 1970 and 2037 or 2440588 <=
jday
<= 2465342 ). The time returned is
localtime (and not GMT).
Parameters
jday
A julian day number between 2440588 and 2465342.
Return Values
The unix timestamp for the start of the given julian day.