.Net时间计算函数，统计某一天是一年的第几周，这一周从哪天开始到哪天结束

 /// <summary>

        /// 计算某年第一周的天数 
        /// </summary> 
        /// <param name="dt">某年中的一个时间</param> 
        /// <returns></returns> 
        public static int DaysInFirstweekInYear(DateTime dt) 
        { 
            DateTime FirstDate = Convert.ToDateTime(string.Format("{0}-1-1", dt.Year)); 
            int DayOfWeekInYear = DayOfWeek(FirstDate); 
            int DaysInFirstWeek = 8 - DayOfWeekInYear; 
            return DaysInFirstWeek; 
        } 

        /// <summary> 
        /// 一年中的第几周 
        /// </summary> 
        /// <param name="dt"></param> 
        /// <returns></returns> 
        public static int WeekOfYear(DateTime dt) 
        { 
            int days = DaysInFirstweekInYear(dt); 
            int dayofyear = dt.DayOfYear; 
            if (dayofyear < days) return 1; 
            else 
            { 
                int week2 = (int)Math.Ceiling((double)(dayofyear - days) / (double)7); 
                return week2 + 1; 
            } 
        } 
        /// <summary> 
        /// 传入某一天，输出这一周的起始日期 
        /// </summary> 
        /// <param name="dt"></param> 
        /// <returns></returns> 
        public static DateTime[] DatesIncludeDay(DateTime dt) 
        { 
            DateTime[] dts = new DateTime[2]; 
            int dayofweek = DayOfWeek(dt); 
            dts[0] = dt.AddDays(1 - dayofweek); 
            dts[1] = dt.AddDays(7 - dayofweek); 
            return dts; 
        } 
        /// <summary> 
        /// 计算星期几，转换为数字 
        /// </summary> 
        /// <param name="dt">某天的日期</param> 
        /// <returns></returns> 
        public static int DayOfWeek(DateTime dt) 
        { 
            string strDayOfWeek = dt.DayOfWeek.ToString().ToLower(); 
            int intDayOfWeek = 0; 
            switch (strDayOfWeek) 
            { 
                case "monday": 
                    intDayOfWeek = 1; 
                    break; 
                case "tuesday": 
                    intDayOfWeek = 2; 
                    break; 
                case "wednesday": 
                    intDayOfWeek = 3; 
                    break; 
                case "thursday": 
                    intDayOfWeek = 4; 
                    break; 
                case "friday": 
                    intDayOfWeek = 5; 
                    break; 
                case "saturday": 
                    intDayOfWeek = 6; 
                    break; 
                case "sunday": 
                    intDayOfWeek = 7; 
                    break; 
            } 
            return intDayOfWeek;

        }

调用：

    DateTime dt = Convert.ToDateTime("2013-1-6");
    Dictionary<string, DateTime> dict = new Dictionary<string, DateTime>();
    dict.Add("0001", DateTime.Now.AddDays(-100));
    dict.Add("0002", DateTime.Now.AddDays(-99));
    dict.Add("0003", DateTime.Now.AddDays(-98));
    dict.Add("0004", DateTime.Now.AddDays(-97));
    dict.Add("0005", DateTime.Now.AddDays(-96));
    dict.Add("0006", DateTime.Now.AddDays(-95));
    dict.Add("0007", DateTime.Now.AddDays(-94));
    dict.Add("0008", DateTime.Now.AddDays(-93));
    dict.Add("0009", DateTime.Now.AddDays(-92));
    dict.Add("0010", DateTime.Now.AddDays(-91));
    dict.Add("0011", DateTime.Now.AddDays(-90));
    dict.Add("0012", DateTime.Now.AddDays(-89));
    dict.Add("0013", DateTime.Now.AddDays(-88));
    dict.Add("0014", DateTime.Now.AddDays(-87));
    dict.Add("0015", DateTime.Now.AddDays(-86));
    dict.Add("0016", DateTime.Now.AddDays(-85));
    dict.Add("0017", DateTime.Now.AddDays(0));
    dict.Add("0018", DateTime.Now.AddDays(1));
    dict.Add("0019", DateTime.Now.AddDays(2));
    string str = "";
    foreach(KeyValuePair<string,DateTime> kv in dict)
    {
        DateTime[] dts = Common.OperateText.DatesIncludeDay(kv.Value);
        str = str + string.Format("{0},第{1}周,时间{2:yyyy-MM-dd},本周从{3:yyyy-MM-dd}到{4:yyyy-MM-dd} <br/>",kv.Key,Common.OperateText.WeekOfYear(kv.Value),kv.Value,dts[0],dts[1]);
    }

Response.Write(str);

效果：