subroutine yd_to_mjdn(d,y,mjdn)
      implicit none
      integer d,y,j,mjdn
c
c subroutine returns the modified julian day number (MJDN) given a date (year,
c day) in the (modern) gregorian calendar.
c MJDN of Nov 17 1858 is 1. This is the day whose Modified
c Julian Date (MJD) spans the range 0.0 - 1.0, MJD being the time in days since
c 00:00 on Nov 17 1858.  MJD is Julian Date - 2400000.5.
c
      j = d+37 + 365*(y+4712) + (y+4715)/4 - (y+4799)/100 + (y+4799)/400
      mjdn=j-2400000
      return
      end
c
c Nov 17 1858 has a Julian Day Number of 2 400 001
c                 a Julian date       of 2 400 000.5 to 2 400 001.5
c                 a MJDN              of 1
c                 a MJ date           of 0.0 to 1.0
c
c So the JDN is the average of the JD for that day (midnight to midnight)
c    the MJDN is the MJD at the end (midnight, 24:00) of the day.
c
c  MJD = MJDN-1 + hours/24.0
c   JD = JDN + (hours-12.0)/24.0      integer function mjdn(d,m,y)
c MJDN = JDN - 2 400 000
c  MJD = JD - 2 400 000.5
c MJDN = int(MJD) + 1
c  JDN = int(JD + 0.5)
c