recurring event calendar inserts

[petrb]

I am using this construct to insert weekly recurring events into a calendar:

switch ($recurring_type) {
case "weekly":

  for( $i=0 ; $i < $recurring_duration ; $i++ ){

    $insmonth = date("m" , $initial_time + $i*7*$SECONDS_PER_DAY);
    $insday   = date("d" , $initial_time + $i*7*$SECONDS_PER_DAY);
    $insyear  = date("Y" , $initial_time + $i*7*$SECONDS_PER_DAY);



   //the db part: $query = "INSERT INTO ... bla bla

    $msg .= "INSERT for " . date("M d Y",$initial_time + $i*7*$SECONDS_PER_DAY) . "<br>\n";
  }
  break;

}

This works fine, but for MONTHLY (say, 2nd Thursday) recurring events, $i47*$SECONDS_PER_DAY only works until there is a month with 5 Thursdays -- How would you loop to find these exceptions?

--OR-- would it be better to completely shift gears and pull new vars from the input form (if so, how):

1st
2nd
3rd
4th... weekday

Thanks,
--ph