asp.net DateTime的.AddDays和.AddHours怎么用？


 //查询所有时间
            List<Tasks> showTasks = TasksManager.SelectAllTasksToIntendingFinishTime(userId, IntendingDateTime);
  //循环遍历所有的时间
            foreach (Tasks tasksIntendingFinishTime in showTasks)
            {
                  //遍历每一条预计完成时间
                DateTime newIntendingFinishTime = tasksIntendingFinishTime.IntendingFinishTime;
 //为周日的情况天数+1
                if (newIntendingFinishTime.DayOfWeek.Equals(DayOfWeek.Sunday))
                {
                    //遇到周日并且时间在上班时间之前
                    if (newIntendingFinishTime.Hour < 9)
                    {
                        //得到时间差值
                        int timeSubtract = 9 - newIntendingFinishTime.Hour;
                        //将时间差值加到所要显示的小时上面
                        newIntendingFinishTime.AddHours(Convert.ToDouble(timeSubtract));
                        //因为是9:00以前并且是周日所以天数+1
                        newIntendingFinishTime.AddDays(Convert.ToDouble(1));
                        //更新时间
                        int upateIrregular = TasksManager.UpdateToIntendingFinishTime(userId,newTaskId, newIntendingFinishTime);
                    }
           }请问我应该怎么写，这样他更新跟没有更新没有任何的区别