<head>
<script language=javascript>
//open pop up windows
function NewWindow( link, h, w, resize, scroll, tool, loc, dir, status, menu ){
var centerWidth=(screen.width / 2) - (w / 2);
var centerHeight=(screen.height / 2) - (h / 2);
window.open(link, "PopUp", "height=" + h + ", width=" + w + ", top=" + centerHeight + ", left=" + centerWidth + ", resizable=" + resize + ", scrollbars=" + scroll + ", toolbar=" + tool + ", location=" + loc + ", directories=" + dir + ", status=" + status + ", menubar=" + menu);
}
</script>
<head>
<body>
<form id="Form1" method="post" runat="server">
<asp:HyperLink id="lnkDate" runat="server" NavigateUrl="javascript: NewWindow('calendar.aspx?frm=Form1&obj=txtDate', 200, 250, 0, 0, 0, 0, 0, 0, 0);">Date</asp:HyperLink>
<asp:textbox id="txtDate" runat="server"></asp:textbox>
</form>
</body>