here's a function that takes 2 arguments: the first is the date (YYYY-DD-MM or most other common date formats will work) and the second is the number days forward you want to calculate:
function date_add ($date, $add)
{
return date ('Y-m-d h:i:s', strtotime ($date) + ($add * 86400));
}
echo date_add ('2003-03-18 10:00:00', 30);