MySQL's date field can only take in strings in the format Y-m-d.
If you want to store a date in mysql, you will have to reformat it into that format.
$_POST['date'] is a field coming in from your calendar in m-d-y form.
$date = date('Y-m-d',strtotime($_POST['date']))
$query = "INSERT INTO table(`date`) VALUES('".$date."')";
(Note: This will also sanitize the date, as strtotime will return -1 if the post value is not a time-string.)