MySql获得更新自上次NOW()异步问题

我遇到了麻烦，我正试图从我的网络聊天的消息表中获取更新。为此，我将最后日期时间存储在会话中，并尝试获取在最后日期时间和现在之间发送的消息。

然而，它缺少一些信息，我不知道为什么。我的mysql日期时间比较有问题吗?

如果我刷新页面，它会重置会话变量并显示所有消息。在javascript中，我每2 +回调秒调用一次web方法。

private string LastNow
{
    get
    {
        return Session["last_now"] as string ?? 
                   (string)(Session["last_now"] = string.Empty);
    }
    set
    {
        Session["last_now"] = value;
    }
}
[WebMethod(EnableSession=true)]
public object GetMessages(int id, int roomId)
{
    string lastDt = LastNow;
    /* EDIT */
    string now = MySqlHelper.ExecuteScalar(
        connstr, "SELECT CAST(NOW() AS CHAR);") as string;
    // if the session variable is null or empty, 
    // get all messages up to now, else most recent
    string dtclause = 
        !string.IsNullOrWhiteSpace(lastDt) ?
        string.Format(" AND sent_dt > '{0}' AND sent_dt <= '{1}'", lastDt, now) :
        string.Format(" AND sent_dt <= '{0}'", now);
    string sql =
        string.Format(
        "SELECT user_id, type, message FROM Chat.Message WHERE room_id={0}{1};",
        roomId, dtclause);
    object ret = null;
    try // with a finally, always store last update time
    {
        using (DataSet msgset = 
               MySqlHelper.ExecuteDataset(Common.SupportDatabase, sql))
        {
            DataTable msgtable = msgset.Tables[0];
            if (msgtable != null)
            {
                DataRow[] rows = msgtable.Select();
                if (rows != null && rows.Length > 0)
                {
                    ret = from msg in rows
                            select new
                            {
                                id = msg["user_id"],
                                type = msg["type"],
                                message = msg["message"]
                            };
                }
            }
        }
    }
    finally
    {
        LastNow = now;                
    }

    return ret;
}