计算当前月底天数

/**//// <summary>
        /// 当前月有多少天
        /// </summary>
        /// <param name="y"></param>
        /// <param name="m"></param>
        /// <returns></returns>
        public static int HowMonthDay(int y, int m)
        {
            int mnext;
            int ynext;
            if (m < 12)
            {
                mnext = m + 1;
                ynext = y;
            }
            else
            {
                mnext = 1;
                ynext = y + 1;
            }
            DateTime dt1 = System.Convert.ToDateTime(y + "-" + m + "-1");
            DateTime dt2 = System.Convert.ToDateTime(ynext + "-" + mnext + "-1");
            TimeSpan diff = dt2 - dt1;
            return diff.Days;
        }

        /**//// <summary>
        /// 得到一年中的某周的起始日和截止日
        /// 年 nYear
        /// 周数 nNumWeek
        /// 周始 out dtWeekStart
        /// 周终 out dtWeekeEnd
        /// </summary>
        /// <param name="nYear"></param>
        /// <param name="nNumWeek"></param>
        /// <param name="dtWeekStart"></param>
        /// <param name="dtWeekeEnd"></param>
        public static void GetWeek(int nYear, int nNumWeek, out   DateTime dtWeekStart, out   DateTime dtWeekeEnd)
        {
            DateTime dt = new DateTime(nYear, 1, 1);
            dt = dt + new TimeSpan((nNumWeek - 1) * 7, 0, 0, 0);
            dtWeekStart = dt.AddDays(-(int)dt.DayOfWeek + (int)DayOfWeek.Monday);
            dtWeekeEnd = dt.AddDays((int)DayOfWeek.Saturday - (int)dt.DayOfWeek + 1);
        }

        /**//// <summary>
        /// 求某年有多少周
        /// 返回 int
        /// </summary>
        /// <param name="strYear"></param>
        /// <returns>int</returns>
        public static int GetYearWeekCount(int strYear)
        {
            string returnStr = "";

            System.DateTime fDt = DateTime.Parse(strYear.ToString() + "-01-01");
            int k = Convert.ToInt32(fDt.DayOfWeek);//得到该年的第一天是周几 
            if (k == 1)
            {
                int countDay = fDt.AddYears(1).AddDays(-1).DayOfYear;
                int countWeek = countDay / 7 + 1;
                return countWeek;

            }
            else
            {
                int countDay = fDt.AddYears(1).AddDays(-1).DayOfYear;
                int countWeek = countDay / 7 + 2;
                return countWeek;
            }

        }

        /**//// <summary>
        /// 求当前日期是一年的中第几周
        /// </summary>
        /// <param name="date"></param>
        /// <returns></returns>
        public static int WeekOfYear(DateTime curDay)
        {
            int firstdayofweek = Convert.ToInt32(Convert.ToDateTime(curDay.Year.ToString() + "- " + "1-1 ").DayOfWeek);

            int days = curDay.DayOfYear;
            int daysOutOneWeek = days - (7 - firstdayofweek);

            if (daysOutOneWeek <= 0)
            {
                return 1;
            }
            else
            {
                int weeks = daysOutOneWeek / 7;
                if (daysOutOneWeek % 7 != 0)
                    weeks++;

                return weeks + 1;
            }
        }