【C#开发中常用方法】日期计算

/// <summary>
/// 获取指定日期，在为一年中为第几周
/// </summary>
/// <param name="dt">指定时间</param>
/// <reutrn>返回第几周</reutrn>
private static int GetWeekOfYear(DateTime dt)
{
   GregorianCalendar gc = new GregorianCalendar();
   int weekOfYear = gc.GetWeekOfYear(dt,CalendarWeekRule.FirstDay, DayOfWeek.Monday);
   return weekOfYear;
}

/// <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>
/// 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;
    }
}