关于一个时间判断的问题~

        string rlt = user.Get_Last_Time(uid);
     
        DateTime dtone = Convert.ToDateTime(rlt);
        DateTime dttwo = Convert.ToDateTime(DateTime.Now);
        TimeSpan span = dttwo.Subtract(dtone);
        int min = Convert.ToInt32(span.Minutes);
        int hour = Convert.ToInt32(span.Hours);

        if (span.TotalMinutes<=30)
        {
            Response.Write("<script>alert('30分钟之内不得在发信息!');location.href='javascript:history.back()'</script>");
        }
        else
        {
            Msg msg = new Msg();
            bool rinfo = msg.Leave_message(msg_info);
            if (rinfo == true)
            {
                Response.Write("<script>alert('发布信息成功!');location.href='index.aspx';</script>");
            }
            else
            {
                Response.Write("<script>alert('发布信息失败!');location.href='javascript:history.back()';</script>");
            }
        }   






    // 获取用户的最后一条留言时间
    public static List<Msg_info> Get_Last_time(int uid)
    {
        SqlConnection conn = DBHelper.Createconn();
        string sql = "SELECT TOP(2) * FROM [Msg_info] WHERE uid=@uid ORDER BY msgid DESC";
        List<SqlParameter> param = new List<SqlParameter>();
        param.Add(new SqlParameter("@uid",uid));

        SqlDataReader reader = DBHelper.Select(sql,param,conn);

        List<Msg_info> msg_infos = new List<Msg_info>();
  
        while (reader.Read())
        {
            Msg_info msg_info = new Msg_info();
            msg_info.adddate = Convert.ToDateTime(reader["adddate"]);
            msg_infos.Add(msg_info);
        }
        
        conn.close();
        return msg_infos;

    }