put the following code at line 1 to convert the entered date to yyyy-mm-dd H:i:s format
<?php
if(isset($_GET["DatumVon"])) $_GET["DatumVon"] = date("Y-m-d H:i:s", strtotime($_GET["DatumVon"]));
if(isset($_GET["DatumBis"])) $_GET["DatumBis"] = date("Y-m-d H:i:s", strtotime($_GET["DatumBis"]));
?>