subroutine yd_to_ymd(idn,iy,im,id)
      implicit none
      integer idn,iy,im,id,i,ly,mosum
      integer molens(12)
      data molens/31,28,31,30,31,30,31,31,30,31,30,31/
c
c subroutine returns the month (m) and day of month (d) given the
c year (y) and day in year (dn).
c
      ly=0
      if(mod(iy,4).eq.0 .and.
     +     (mod(iy,100).ne.0 .or. mod(iy,400).eq.0) )ly=1
      molens(2)=28+ly
      im=1
      mosum=0
      id=idn
      do i=1,12
        mosum=mosum+molens(i)
        if(idn.gt.mosum)then
          id=idn-mosum
          im=i+1
        endif
      enddo
      return
      end