to convert the date back to yyyy-mm-dd format, you need to add code using the date and strtotime functions.
for example sakes, lets say the date form element is named "dateField". you would add code at line 1 of the update page to see if dateField has a value and convert it:
<?php
if(isset($_POST['dateField'])) {
$_POST['dateField'] = date("Y-m-d", strtotime($_POST['dateField']));
}
?>