获取某月当天的时间（凌晨和夜晚）

  /// <summary>

        /// 取得某月的第一天

        /// </summary>

        /// <param name="datetime">要取得月份第一天的时间</param>

        /// <returns></returns>

        private DateTime FirstDayOfMonth()

        {

           DateTime datetime=Convert.ToDateTime(DateTime.Now.ToString("yyyy-MM-dd 00:00:00"));

           return datetime.AddDays(1 - datetime.Day);

        }

        /**/

        /// <summary>

        /// 取得某月的最后一天

        /// </summary>

        /// <param name="datetime">要取得月份最后一天的时间</param>

        /// <returns></returns>

        private DateTime LastDayOfMonth()

        {

            DateTime datetime = Convert.ToDateTime(DateTime.Now.ToString("yyyy-MM-dd 23:59:59"));

            return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);

        }