If you know the format of date in $_POST[intake_date] you can use explode to get year , month and time and then concatenate to form a valid mySql date. for example if you are getting something like 12/15/1988 in date you can do like this
$date = explode($_POST['intake_date'], '/');
mysql_date = $date[2].'-'$date[1].'-'$date[0];
though if you have valid date date('y-m-d', strtotime($date)); should also work