c
c  Subroutine to compute pertual days from a date
c  YYYY = year > 1960 
c  Revised to convert either way - May 30, 2006
c
      Subroutine perpd2(idat,kperp)
      integer iyr,imon,iday,kperp,mcum(12),mind(12),days
      Data mcum/0,31,59,90,120,151,181,212,243,273,304,334/
      Data mind/31,29,31,30,31,30,31,31,30,31,30,31/

      if(kperp.lt.1)then
       iyr = idat/10000
       jdat = idat - iyr*10000
       imon = jdat/100
       iday = jdat - imon*100

      if(iyr.lt.1960)go to 99
      kperp = 0
      iyrm=iyr-1
      if(iyrm.lt.1960)go to 10
      do 5 i=1960,iyrm
        k = mod(i,4)
        if(k.ne.0)then
           kperp = kperp + 365
         else
           kperp = kperp + 366
         endif
 5    continue

 10   if(imon.lt.1)go to 99
        if(imon.gt.12)go to 99
         if(iday.lt.1)go to 99
           if(iday.gt.mind(imon))go to 99

          k = mod(iyr,4)
         ky = 0
        if(k.eq.0)ky = 1
      days = mcum(imon)
      if(imon.gt.2)days = days + ky
      kperp = kperp + days + iday
 99   continue

      else
c
c  convert perpetual days back to yyyymmdd
c
      ky = 1959
      jp = kperp
 50   ky = ky + 1
        jsub = 365
         if(mod(ky,4).eq.0)jsub = 366
         jp = jp - jsub
          if(jp.gt.0)go to 50
           iyr = ky
            jp = jp + jsub
          lp =0
         if(mod(iyr,4).eq.0)lp=1
      km = 0  
 30   km = km + 1
        jsub = mind(km)
         if(km.eq.2.and.lp.eq.0)jsub= mind(km) - 1
          jp = jp - jsub
           if(jp.gt.0)go to 30
            imon = km
             jp = jp + jsub
      
        iday = jp
       idat = iyr*10000 + imon*100 + iday
      endif   
      return
      end