!$Author$
!$Date$
!$Revision$
!$HeadURL$

      function julday( dd, mm, yyyy ) result( julian_day )

!     + + + PURPOSE + + +
!     In this routine JULDAY returns the Julian Day Number which begins at
!     noon of the gregorian calendar date specified by day "dd", month "mm", & year "yyyy"
!     All are integer variables. Positive year signifies A.D.; zero and negative, B.C.
!     Calendar dates before 1582 will not match dates on the Julian calendar used
!     at the time.

!     Based on info from http://en.wikipedia.org/wiki/Julian_day, which references
!     http://www.astro.uu.nl/~strous/AA/en/reken/juliaansedag.html, the code
!     was revised to use the Astronomical Gregorian calendar, which takes the
!     present pattern of leap years back into the past. This is ideal for
!     our purposes with no year getting short changed. Integer math method
!     is taken from Wikipedia article.

!     + + + ARGUMENT DECLARATIONS + + +
      integer, intent(in) :: dd     ! integer value of day in the range 1-31
      integer, intent(in) :: mm     !                month in the range 1-12
      integer, intent(in) :: yyyy   !                 year (negative B.C., positive A.D.)

      integer :: julian_day

      julian_day = (1461 * (yyyy + 4800 + (mm - 14)/12))/4              &
     &       + (367 * (mm - 2 - 12 * ((mm - 14)/12)))/12                &
     &       - (3 * ((yyyy + 4900 + (mm - 14)/12)/100))/4 + dd - 32075

      end function julday