<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="viewport" content="width=device-width, initial-scale=1">
<!--jQuery-->
<script src="http://code.jquery.com/jquery-1.10.2.min.js"></script>
<!--jQuery UI-->
<script src="http://view.jqueryui.com/master/ui/datepicker.js"></script>
<!--jQuery Mobil-->
<link rel="stylesheet" href="http://code.jquery.com/mobile/1.4.2/jquery.mobile-1.4.2.min.css">
<script src="http://code.jquery.com/mobile/1.4.2/jquery.mobile-1.4.2.min.js"></script>
<!-- jQuery Mobile Datepicker -->
<link href="http://rawgithub.com/arschmitz/jquery-mobile-datepicker-wrapper/master/jquery.mobile.datepicker.css" rel="stylesheet">
<script src="http://rawgithub.com/arschmitz/jquery-mobile-datepicker-wrapper/master/jquery.mobile.datepicker.js"></script>
</head>
<body>
<div data-role="page" id="pageone">
<div data-role="header">
<h1>Start</h1>
</div>
<div data-role="main" class="ui-content">
<form action="this.php" method="post" data-ajax="true">
<label for="gebdat">Date</label>
<input type="text" name="gebdat" data-role="date" data-date-format="yy-mm-dd" >
<button type="submit" name="submit" class="ui-btn-hidden" data-icon="edit" aria-disabled="false">Save</button>
</form>
</div>
<div data-role="footer">
<h1>End</h1>
</div>
</div>
</body>
</html>
With data-date-format="yy-mm-dd"
you set the format and not with jQuery.