function pn_month {
  typeset ym=$1 pn=$2 x n
  (( x = ym % 100 + pn ))

  if (( x > 0 ))
  then (( n = (x-1) / 12 ))
  else (( n = - (12-x) / 12 ))
  fi
  
  printf "%s\n" $(( ym + pn + 88*n ))
}