you have conflicting version of jquery on the page
<script type="text/javascript" src="../js/jquery-1.8.0.min.js"></script>
<link type="text/css" href="../webassist/datepicker/insert-news_datepicker_1/jquery-ui.css" rel="stylesheet" />
<script type="text/javascript" src="../webassist/jquery/jquery.js"></script>
<script type="text/javascript" src="../webassist/jquery/jquery-ui.js"></script>
before the datepicker, you include js/jquery-1.8.0.min.js
then there is the webassist jquery version:
/webassist/jquery/jquery.js
these are conflicting with each other.