facebook scheduled_publish_time need help

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C#

ASP

string app_id = "?????????";
      string app_secret = "?????????";
      string scope = "publish_stream,manage_pages";

      if (Request["code"] == null)
      {
          Response.Redirect(string.Format(
              "https://graph.facebook.com/oauth/authorize?client_id={0}&redirect_uri={1}&scope={2}",
              app_id, Request.Url.AbsoluteUri, scope));
      }
      else
      {
          Dictionary<string, string> tokens = new Dictionary<string, string>();

          string url = string.Format("https://graph.facebook.com/oauth/access_token?client_id={0}&redirect_uri={1}&scope={2}&code={3}&client_secret={4}",
              app_id, "http://localhost:62057/FacebookShareLink/Default.aspx", scope, Request["code"].ToString(), app_secret);
          //"http://localhost:49252/WebSite4/Default.aspx"
          HttpWebRequest request = System.Net.WebRequest.Create(url) as HttpWebRequest;

          using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
          {
              StreamReader reader = new StreamReader(response.GetResponseStream());

              string vals = reader.ReadToEnd();

              foreach (string token in vals.Split('&'))
              {
                  //meh.aspx?token1=steve&token2=jake&...
                  tokens.Add(token.Substring(0, token.IndexOf("=")),
                      token.Substring(token.IndexOf("=") + 1, token.Length - token.IndexOf("=") - 1));
              }
          }

          string access_token = tokens["access_token"];

          var client = new FacebookClient(access_token);

          dynamic parameters = new ExpandoObject();
          parameters.message = Session["postmsg"].ToString();
          parameters.link = "http://www.filmatest.somee.com";
          parameters.picture = "http://www.filmatest.somee.com/Image_Bg/head.gif";
          parameters.name = "Article Title";
          parameters.caption = "Caption for the link";
          //parameters.published = false;
          parameters.scheduled_publish_time = Facebook.DateTimeConvertor.ToUnixTime(DateTime.Now.AddMinutes(15)); ;

          //parameters.scheduled_publish_time = GetUnixTimestamp(DateTime.Now.AddMinutes(20));


          client.Post("/Look4Movie/feed", parameters);
          Session.Clear();
          Session.Abandon();

          Response.Redirect("http://localhost:62057/FacebookShareLink/Default.aspx");
      }

this is how i try to post on facebook fanpage when i take off parameters.scheduled_publish_time this work fine but when i try to use it i get this error
(OAuthException - #100) (#100) You cannot specify a scheduled publish time on a published post c#
can some one help me plss?
sorry for my bad english