CAL2JD.f90

 	SUBROUTINE CAL2JD (IY0,IM0,ID0,DJM,J)
*  - - - - - - - - - - -
*   i a u _ C A L 2 J D
*  - - - - - - - - - - -
*  Gregorian Calendar to Julian Date.
*
*  Given:
*     IY,IM,ID    i     year, month, day in Gregorian calendar (Note 1)
*
*  Returned:
*     DJM0        d     MJD zero-point: always 2400000.5
*     DJM         d     Modified Julian Date for 0 hrs
*     J           i     status:
*                           0 = OK
*                          -1 = bad year   (Note 3: JD not computed)
*                          -2 = bad month  (JD not computed)
*                          -3 = bad day    (JD computed)
*-----------------------------------------------------------------------

      IMPLICIT NONE

      INTEGER IY0, IM0, ID0,IY, IM, ID
      real*8 DJM0, DJM
      INTEGER J, MY, IYPMY

*  Earliest year allowed (4800BC)
      INTEGER IYMIN
      PARAMETER ( IYMIN = -4799 )

*  Month lengths in days
      INTEGER MTAB(12)
      DATA MTAB / 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 /

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
      IY=IY0;IM=IM0;ID=ID0
*  Preset status.
      J = 0

*  Validate year.
      IF ( IY.LT.IYMIN ) THEN
         J = -1
      ELSE

*     Validate month.
         IF ( IM.GE.1 .AND. IM.LE.12 ) THEN

*        Allow for leap year.
            IF ( MOD(IY,4) .EQ. 0 ) THEN
               MTAB(2) = 29
            ELSE
               MTAB(2) = 28
            END IF
            IF ( MOD(IY,100).EQ.0 .AND. MOD(IY,400).NE.0 ) MTAB(2) = 28

*        Validate day.
            IF ( ID.LT.1 .OR. ID.GT.MTAB(IM) ) J = -3

*        Result.
            MY = ( IM - 14 ) / 12
            IYPMY = IY + MY
            DJM0 = 2400000.5D0
            DJM = DBLE( ( 1461 * ( IYPMY + 4800 ) ) / 4
     :                + (  367 * ( IM-2 - 12*MY ) ) / 12
     :                - (    3 * ( ( IYPMY + 4900 ) / 100 ) ) / 4
     :                + ID - 2432076)
            DJM=DBLE(nint(DJM*1.d3))*1.d-3
*        Bad month
         ELSE
            J = -2
         END IF
      END IF

      END