Content '\/Date('')\/' does not start with '\/Date(' and end with ')\/' as required for JSON

I want to send a post request to a WCF rest service as you can see:

Guid id;
id = Guid.NewGuid();

var httpWebRequest = (HttpWebRequest)WebRequest.Create("http://localhost:47026/NewsRepository.svc/AddNews");
httpWebRequest.ContentType = "application/json";
httpWebRequest.Method = "POST";
using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
{
    string json = new JavaScriptSerializer().Serialize(new
    {       
        id = id,
        Subject="wfwf",
        ViewerCounter="1",
        Content="fsdsd",
        SubmitDatatime="2012/12/12",
        ModifiedDateTime="2012/12/12",
        PublisherName="sdaadasd",
        PictureAddress="adfafsd",
        TypeOfNews="adsadaad"                       
    });

    streamWriter.Write(json);
}
try 
{ 
    var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
    using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
    {
        var result = streamReader.ReadToEnd();
    }
}
catch(Exception er)
{
    MessageBox.Show(er.ToString());
}

But I get an error - 400 bad request. So I traced my WCF log file and I found this error:

There was an error deserializing the object of type CMSManagement.Domain.Entity.News. DateTime content '2012/12/12' does not start with '/Date(' and end with ')/' as required for JSON.

After my dear friend @svek code. The result is like this:

Guid id;
id = Guid.NewGuid();

var httpWebRequest = (HttpWebRequest)WebRequest.Create("http://localhost:47026/NewsRepository.svc/AddNews");
httpWebRequest.ContentType = "application/json";
httpWebRequest.Method = "PUT";
using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
{
    JsonSerializerSettings microsoftDateFormatSettings = new JsonSerializerSettings
    {
        DateFormatHandling = DateFormatHandling.MicrosoftDateFormat
    };

    string json = JsonConvert.SerializeObject(new
    {
        id = id,
        Subject = "wfwf",
        ViewerCounter = "1",
        Content = "fsdsd",
        SubmitDatatime = "2012/12/12",
        ModifiedDateTime = "2012/12/12",
        PublisherName = "sdaadasd",
        PictureAddress = "adfafsd",
        TypeOfNews = "adsadaad"
    }, microsoftDateFormatSettings);
    streamWriter.Write(json);
}
try 
{ 
    var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
    using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
    {
        var result = streamReader.ReadToEnd();
    }
}
catch(Exception er)
{
    MessageBox.Show(er.ToString());
}

But I get the same error. Why?