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);
        }