[Gasconade]

With PHP you can do something like:

if($mydate_from_db > $present_date){
    $mydate_from_db = strtotime($mydate_from_db, '+ 10 days');
} else {
    $mydate_from_db = strtotime($present_date, '+ 10 days');
}

Or you can do it with mysql like:

UPDATE yourtable SET mydate = IF(mydate = '0000-00-00 00:00:00', '2013-06-18 00:00:00', DATE_ADD(mydate, INTERVAL 10 DAY));