search for maths on working out percentages. no joke i did.
$today = new DateTime();
$startdate = new DateTime("2013-06-16"); //$model['StartDATE']
$enddate = new DateTime("2013-06-26"); // $model['ProjectEndDate']
$diff_total = $enddate->diff($startdate)->format("%a");
$diff_today = $today->diff($startdate)->format("%a");
$percentage_date=round(($diff_today/$diff_total)*100,2);
echo $percentage_date.'%'; // from today
get percentage of a date
$percentage_to_get = 45;
$percentage_of_days = floor(($diff_total/100*)$percentage_to_get);
echo date('Y-m-d', strtotime( $startdate->format('Y-m-d') ." + $percentage_of_days day") );