Try something like this:
$Fifteendays = 60 * 60 * 24 * 15;
//Seconds X Minutes X Hours X Days
$Infifteendays = time() + $Fifteendays; //Unix time
//Add it to the current time
echo $Infifteendays;
//Or in 'Pretty' time
echo date("M d Y H:i:s", $Infifteendays);