Dev DateEdit顯示本月第一天和最後一天

         void ShowFirstAndEndDayOnDate()
        {
            this.dateStartTime.Text = DateTime.Now.AddDays(-DateTime.Now.Day + 1).ToString("yyyy-MM-dd");
            this.dateEndTime.Text = DateTime.Now.AddMonths(1).AddDays(-DateTime.Now.AddMonths(1).Day).ToString("yyyy-MM-dd");
        }