if you're in mysql you can create a timestamp field and then index it, this will make it change automatically when a record is added.
However mysql and php timestamps are different so you'll probably find this function handy if you do that:
<?php
function calc_working_date($start,$num) {
$months = array('January','February','March','April','May','June','July','August','September','October','November','December');
$weekend = array('sat','sun');
$date_ary = explode('-',$start);
$start = $months[intval($date_ary[0])-1] . ' ' . $date_ary[1] . ', ' . $date_ary[2];
$start_time = strtotime($start);
$cur_time = $start_time;
$i = 0;
while($i < $num) {
if(!in_array(strtolower(date('D',$cur_time)),$weekend)) {
$i++;
}
$cur_time = $cur_time + 86400;
} //end while
$retval = date('n-d-Y',$cur_time);
return $retval;
} //end calc_working_days
?>