!***********************************************************************************************************************************
!
!                                                          G R E G 2 J D
!
!
!  Program:      GREG2JD
!
!  Programmer:   Dr. David G. Simpson
!                Prince George's Community College
!                Largo, Maryland  20774
!
!  Date:         November 20, 2001
!
!  Language:     Fortran-90
!
!  Version:      1.00b  (October 25, 2004)
!
!  Description:  This program converts a date on the Gregorian or Julian calendars to a Julian day.
!
!***********************************************************************************************************************************

!***********************************************************************************************************************************
!  Main program
!***********************************************************************************************************************************

      PROGRAM GREG2JD

      IMPLICIT NONE

      INTEGER :: A, B                                                               ! intermediate variables
      DOUBLE PRECISION :: D                                                         ! day of month (+ fraction)
      DOUBLE PRECISION :: JD                                                        ! Julian day
      INTEGER :: M                                                                  ! month (1-12)
      INTEGER :: Y                                                                  ! year



      WRITE (UNIT=*, FMT='(A)', ADVANCE='NO') ' Enter month (1-12):  '              ! prompt for month
      READ (UNIT=*, FMT=*) M

      WRITE (UNIT=*, FMT='(A)', ADVANCE='NO') ' Enter day:  '                       ! prompt for day of month
      READ (UNIT=*, FMT=*) D

      WRITE (UNIT=*, FMT='(A)', ADVANCE='NO') ' Enter year:  '                      ! prompt for year
      READ (UNIT=*, FMT=*) Y

      IF (M .LE. 2) THEN                                                            ! begin algorithm
         Y = Y - 1
         M = M + 12
      END IF

      A = Y/100
      B = 2 - A + A/4

      JD = INT(365.25D0*(Y+4716)) + INT(30.6001D0*(M+1)) + D + B - 1524.5D0         ! end of algorithm

      WRITE (UNIT=*, FMT='(/,A, F15.6)') ' Julian day = ', JD                       ! print result

      END PROGRAM GREG2JD