the format stored in the database must by yyyy-dd-mm
you can use the date() and strtotime() functions to allow you to enter the date using dd.mm.yyyy format and have it stored correctly, for exaample add code similar to this at line one of the data assist pages:
<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
$_POST['dateElement'] = "".((isset($_POST["dateElement"]))?date("Y-m-d", strtotime($_POST["dateElement"])):"") ."";
}
?>
and then use code similar to this to convert it back when displaying:
<?php echo(date("d.m.Y", strtotime($row_recordsetName['dateColumn']))); ?>