C# 根据年、月、周、星期获得日期等

/// <summary> 
/// 取指定日期是一年中的第几周 
/// </summary> 
/// <param name="dtime">给定的日期</param> 
/// <returns>数字 一年中的第几周</returns> 
public static int WeekOfYear(DateTime dtime)
{
    try
    {
        //确定此时间在一年中的位置
        int dayOfYear = dtime.DayOfYear;
        //当年第一天
        DateTime tempDate = new DateTime(dtime.Year, 1, 1);
        //确定当年第一天
        int tempDayOfWeek = (int)tempDate.DayOfWeek;
        tempDayOfWeek = tempDayOfWeek == 0 ? 7 : tempDayOfWeek;
        ////确定星期几
        int index = (int)dtime.DayOfWeek;
        index = index == 0 ? 7 : index;

        //当前周的范围
        DateTime retStartDay = dtime.AddDays(-(index - 1));
        DateTime retEndDay = dtime.AddDays(6 - index);

        //确定当前是第几周
        int weekIndex = (int)Math.Ceiling(((double)dayOfYear + tempDayOfWeek - 1) / 7);

        if (retStartDay.Year < retEndDay.Year)
        {
            weekIndex = 1;
        }

        return weekIndex;
    }
    catch (Exception ex)
    {
        throw new Exception(ex.Message);
    }

}

/// <summary>
/// 求某年有多少周
/// </summary>
/// <param name="dtime"></param>
/// <returns></returns>
public static int GetWeekOfYear(DateTime dtime)
{
    int countDay = DateTime.Parse(dtime.Year + "-12-31").DayOfYear;
    int countWeek = countDay / 7;
    return countWeek;
}


//根据年月日获得星期几
public static string CaculateWeekDay(int month, int day)
{
    string weekstr = string.Empty;
    int year = DateTime.Now.Year;
    //把一月和二月看成是上一年的十三月和十四月
    if (month == 1) { month = 13; year--; }
    if (month == 2) { month = 14; year--; }
    int week = (day + 2 * month + 3 * (month + 1) / 5 + year + year / 4 - year / 100 + year / 400) % 7;
    switch (week)
    {
        case 0: weekstr = "1"; break;
        case 1: weekstr = "2"; break;
        case 2: weekstr = "3"; break;
        case 3: weekstr = "4"; break;
        case 4: weekstr = "5"; break;
        case 5: weekstr = "6"; break;
        case 6: weekstr = "7"; break;
    }
    return weekstr;
}

/// <summary>
/// 根据2个时间段获得相应的周数
/// </summary>
/// <param name="beginDate"></param>
/// <param name="endDate"></param>
/// <returns></returns>
public static int WeekOfDate(DateTime beginDate, DateTime endDate)
{
    TimeSpan ts1 = new TimeSpan(beginDate.Ticks);
    TimeSpan ts2 = new TimeSpan(endDate.Ticks);
    TimeSpan ts = ts2.Subtract(ts1).Duration();
    int weeks = ts.Days / 7;

    //确定此时间在一年中的位置
    int dayOfYear = beginDate.DayOfYear;
    //当年第一天
    DateTime tempDate = new DateTime(beginDate.Year, beginDate.Month, beginDate.Day);
    //最后一天
    DateTime tempendDate = new DateTime(endDate.Year, endDate.Month, endDate.Day);
    int tempDayOfWeek = (int)tempDate.DayOfWeek;
    tempDayOfWeek = tempDayOfWeek == 0 ? 7 : tempDayOfWeek;

    ////确定星期几
    int startindex = (int)beginDate.DayOfWeek;
    startindex = startindex == 0 ? 7 : startindex;
    //当前周的范围
    DateTime retStartDay = beginDate.AddDays(-(startindex - 1));
    DateTime retEndDay = beginDate.AddDays(7 - startindex);
    //确定当前是第几周
    int weekIndex = (int)Math.Ceiling(((double)dayOfYear + tempDayOfWeek - 1) / 7);

    return weeks;
}

/// <summary>
/// 根据起始时间，获取第几周
/// </summary>
/// <param name="dtime">当前时间</param>
/// <returns></returns>
public static int WeekOfTermDate(DateTime dtime)
{          
    string datetime = "2011-3-1";

    TimeSpan ts1 = new TimeSpan(dtime.Ticks);
    TimeSpan ts2 = new TimeSpan(Convert.ToDateTime(datetime).Ticks);
    TimeSpan ts = ts2.Subtract(ts1).Duration();

    //确定此时间在一年中的位置
    int dayOfYear = ts.Days;
    //当年第一天
    DateTime tempDate = new DateTime(Convert.ToDateTime(datetime).Year, Convert.ToDateTime(datetime).Month, Convert.ToDateTime(datetime).Day);
   
    int tempDayOfWeek = (int)tempDate.DayOfWeek;
    tempDayOfWeek = tempDayOfWeek == 0 ? 7 : tempDayOfWeek;
    ////确定星期几
    int index = (int)dtime.DayOfWeek;
    index = index == 0 ? 7 : index;

    //当前周的范围
    DateTime retStartDay = dtime.AddDays(-(index - 1));
    DateTime retEndDay = dtime.AddDays(7 - index);

    //确定当前是第几周
    int weekIndex = (int)Math.Ceiling(((double)dayOfYear + tempDayOfWeek) / 7);          
    return weekIndex;
}

/// <summary>
/// 根据周，星期获得具体年月日
/// </summary>
/// <param name="week">第几周</param>
/// <param name="day">星期几</param>
/// <returns></returns>
public static DateTime DateTimeByWeekAndDay(int week, int day)
{           
    DateTime someTime = Convert.ToDateTime("2011-3-1");

    int i = someTime.DayOfWeek - DayOfWeek.Monday;  
    if (i == -1) i = 6;// i值 > = 0 ，因为枚举原因，Sunday排在最前，此时Sunday-Monday=-1，必须+7=6。
    TimeSpan ts = new TimeSpan(i, 0, 0, 0);          

    //获取第N周的星期一
    someTime = someTime.Subtract(ts).AddDays((week - 1) * 7);
    //获得星期几
    someTime = someTime.AddDays(day - 1);
    return someTime;
}