In your UpdatePanel add css class on your test box
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:TextBox ID="TextBox1" runat="server" CssClass="classTarget"></asp:TextBox>
.....
</ContentTemplate>
</asp:UpdatePanel>
In your script add this code
<script type="text/javascript" src="http://jqueryjs.googlecode.com/files/jquery-1.3.1.js">
</script>
<script type="text/javascript">
$(document).ready(function() {
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
function EndRequestHandler(sender, args) {
$('.classTarget).datepicker({ dateFormat: 'dd-mm-yy' });
}
});
</script>