Your method returns JsonResult. This is MVC specific and you cannot use it in a webforms application.

If you want to call methods in the code behind in a classic WebForms application you could use PageMethods:

[WebMethod]
public static string GetDate()
{
    return DateTime.Now.ToString();
}

And then to call the method:

$.ajax({
    type: 'POST',
    url: 'PageName.aspx/GetDate',
    data: '{ }',
    contentType: 'application/json; charset=utf-8',
    dataType: 'json',
    success: function(msg) {
        // Do something interesting here.
    }
});

And here's a full working example I wrote for you:

<%@ Page Language="C#" %>
<%@ Import Namespace="System.Web.Services" %>
<script type="text/C#" runat="server">
    [WebMethod]
    public static string SayHello(string name)
    {
        return "Hello " + name;
    }
</script>
<!DOCTYPE html>
<html>
<head>
    <title></title>
    <script type="text/javascript" src="/scripts/jquery-1.4.1.js"></script>
    <script type="text/javascript">
        $(function () {
            $.ajax({
                type: 'POST',
                url: 'default.aspx/sayhello',
                data: JSON.stringify({ name: 'John' }),
                contentType: 'application/json; charset=utf-8',
                dataType: 'json',
                success: function (msg) {
                    // Notice that msg.d is used to retrieve the result object
                    alert(msg.d);
                }
            });
        });
    </script>
</head>
<body>
    <form id="Form1" runat="server">

    </form>
</body>
</html>

PageMethods are not limited to simple argument types. You could use any type as input and output, it will be automatically JSON serialized.