caldate Subroutine

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 Calculates the Gregorian date from the Julian date                     *
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 AUTHOR: Andreas Stohl (21 January 1994), adapted from Numerical Recipes*
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 PS 2020-07-27: add a check to avoid giving back 240000 for hhmiss      *
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 Variables:                                                             *
 dd             Day                                                     *
 hh             Hour                                                    *
 hhmiss         Hour, Minute, Second                                    *
 ja,jb,jc,jd,je help variables                                          *
 jalpha         help variable                                           *
 juliandate        Julian Date                                          *
 julday         help variable                                           *
 mi             Minute                                                  *
 mm             Month                                                   *
 ss             Seconds                                                 *
 yyyy           Year                                                    *
 yyyymmdd       Year, Month, Day                                        *
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 Constants:                                                             *
 igreg          help constant                                           *
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