YYMMDD is a valid mysql date format, so if you knew you would always get a date in this format from excel (5 or 6 numbers), you could reverse that excel date.
$date='30503';
$month=substr($date, 0, strlen($date)-4);
$day=substr($date, -4, 2);
$year=substr($date, -2);
if(strlen($month)==1) $month='0'.$month;
$mysqldate= $year.$month.$day;
echo $mysqldate;