when i view source of your page, i see that you are including the jquery library in this script line:
<script type="text/javascript" src="jquery.js"></script>
this attaches to a jquery file from version 1.3.2
further up the page, the date picker adds another jquery referance:
<script type="text/javascript" src="webassist/jquery/jquery.js"></script>
this one is for version 1.4.2.
This is causing a conflict and is breaking the date picker.
you should remove the script reference to the older jquery file