function pn_weekday {
  typeset ymd=$1 weekday=$2 pn=${3:-0} i x found=0 IN=0
  [[ $ymd == +* ]] && IN=1

    if (( pn < 0 ))
  then (( sign = -1 ))
  elif (( pn > 0 ))
  then (( sign = +1 ))
  else (( sign =  0 ))
  fi

  (( i = pn*sign*7 ))

  while (( i > 0 )); do
    (( IN == 0 )) && ymd=$(pn_day $ymd $sign)

    if [[ -z $x ]]
    then x=$(cur_weekday $ymd)
    else (( x = (x+1) % 7 ))
    fi

    (( x == weekday )) && {
      (( found = ymd ))
    }

    (( IN == 1 )) && ymd=$(pn_day $ymd $sign)
    (( i = i -1 ))
  done

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