function pn_day_nr {
  typeset ymd=$1 pn=${2:-0}

  function pn_day1 {
    typeset ymd=$1 pn=$2 d ym x
    ((  d = ymd % 100 ))
    (( ym = ymd / 100 ))
    if (( pn == -1 )); then
      if (( d > 1 )); then
        (( x = ymd - 1 ))
        (( x > 17520902 && x < 17520914 )) && (( x = 17520902 ))
      else
        x=$(end_month $(pn_month $ym -1))
      fi
    elif (( pn == +1 )); then
      if (( d < 28 )) || (( ymd < $(end_month $ym) )); then
        (( x = ymd + 1 ))
        (( x > 17520902 && x < 17520914 )) && (( x = 17520914 ))
      else
        x=$(( 100*$(pn_month $ym +1) + 1 ))
      fi
    fi

    printf "%s\n" $x
    return 0
  }

  while (( pn != 0 )); do
    if (( pn < 0 ))
    then ymd=$(pn_day1 $ymd -1); (( pn = pn + 1 ))
    else ymd=$(pn_day1 $ymd +1); (( pn = pn - 1 ))
    fi
  done

  printf "%s\n" $ymd
  return 0
}