.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：