integer function mjdn(d,m,y)
      integer d,m,y,j
c
c function returns the modified julian day number (MJDN) given a date (year,
c month, day) in the (modern) gregorian calendar. Does not work for years
c less than 0 (2 BC or earlier).
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= 367*y - 7*(y+(m+9)/12)/4 - 3*((y+(m-9)/7)/100+1)/4
     +             + 275*m/9 + d + 1721029
c      mjdn=j
      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
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