I am using a proxy service to allow my client side javascript to talk to a service on another domain

The proxy is a simple ashx file with simply gets the request and forwards it onto the service on the other domain :

using (var sr = new System.IO.StreamReader(context.Request.InputStream))
            {
                requestData = sr.ReadToEnd();
            }

            string data = HttpUtility.UrlDecode(requestData);

            using (var client = new WebClient())
            {
                client.BaseAddress = serviceUrl;
                client.Headers.Add("Content-Type", "application/json");

                response = client.UploadString(new Uri(webserviceUrl), data);
            }

The client javascript calling this proxy looks like this

function TestMethod() {

    $.ajax({
        type: "POST",
        url: "/custommodules/configuratorproxyservice.ashx?m=TestMethod",
        contentType: "application/json; charset=utf-8",
        data: JSON.parse('{"testObj":{"Name":"jo","Ref":"jones","LastModified":"\/Date(-62135596800000+0000)\/"}}'),
        dataType: "json",
        success: AjaxSucceeded,
        error: AjaxFailed
    });

    function AjaxSucceeded(result) {
        alert(result);
    }

    function AjaxFailed(result) {
        alert(result.status + ' - ' + result.statusText);
    }
}

This works fine until I have to pass a date. At which point I get a Bad Request error when the proxy tries to call the service

I did have this working at one point but have now lost it.

Have tried using JSON.Parse on the object before sending. and JSON.Stringify, but no joy

anyone got any ideas what I am missing