you have conflicting versions of jquery on the page.
the date picker adds these lines for jquery:
<link type="text/css" href="../webassist/datepicker/order_datepicker_1/jquery-ui.css" rel="stylesheet" />
<script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
further down you add it again:
<script src="../js/jquery.min.js"></script>
only one instance of jquery can be added to the page