ASP.Net Error: The given path's format is not supported

how to custom name of ErrorLogFile using c#

whats wrong mention code

private void LogError(Exception ex)
{
    string message = string.Format("Time: {0}", dt.ToString("dd/MM/yyyy hh:mm:ss tt"));
    message += Environment.NewLine;
    message += "-----------------------------------------------------------";
    message += Environment.NewLine;
    message += string.Format("Message: {0}", ex.Message);
    message += Environment.NewLine;
    message += string.Format("StackTrace: {0}", ex.StackTrace);
    message += Environment.NewLine;
    message += string.Format("Source: {0}", ex.Source);
    message += Environment.NewLine;
    message += string.Format("TargetSite: {0}", ex.TargetSite.ToString());
    message += Environment.NewLine;
    message += "-----------------------------------------------------------";
    message += Environment.NewLine;
    string filename = "ErrorLog"+dt+"";
    string path = Server.MapPath("~/ErrorLog/"+ filename + ".txt");
    using (StreamWriter writer = new StreamWriter(path, true))
    {
        writer.WriteLine(message);
        writer.Close();
    }
}

gives error 

System.NotSupportedException: 'The given path's format is not supported.'