日期算法公式

注：一下方法的参数都为本地系统时间，即:datetime.now

/// <summary>
/// 计算本周起始日期（礼拜一的日期）
/// </summary>
/// <param name="someDate">该周中任意一天</param>
/// <returns>返回礼拜一日期，后面的具体时、分、秒和传入值相等</returns>
public static DateTime CalculateFirstDateOfWeek(DateTime someDate)
{
     int i = someDate.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);
     return someDate.Subtract(ts).Date;
}

/// <summary>
/// 计算本周结束日期（礼拜五的日期）
/// </summary>
/// <param name="someDate">该周中任意一天</param>
/// <returns>返回礼拜日日期，后面的具体时、分、秒和传入值相等</returns>
public static DateTime CalculateLastDateOfWeek(DateTime someDate)
{
    int i = someDate.DayOfWeek - DayOfWeek.Sunday;
    if (i != 0) i = 7 - i;// 因为枚举原因，Sunday排在最前，相减间隔要被7减。
    TimeSpan ts = new TimeSpan(i, 0, 0, 0);
    //return someDate.Add(ts).AddDays(1).Date;
    return someDate.Add(ts).AddDays(-2).AddHours(24).AddSeconds(-1).Date;
}
/// <summary>
/// 判断选择的日期是否是本周（根据系统当前时间决定的‘本周'比较而言）
/// </summary>
/// <param name="someDate"></param>
/// <returns></returns>
public static bool IsThisWeek(DateTime someDate)
{
     //得到someDate对应的周一
      DateTime someMon = CalculateFirstDateOfWeek(someDate);
     //得到本周一
    DateTime nowMon = CalculateFirstDateOfWeek(DateTime.Now);
    TimeSpan ts = someMon - nowMon;
    if (ts.Days < 0)
    ts = -ts;//取正
    if (ts.Days >= 7)
   {
     return false;
   }
   else
   {
     return true;
   }
   }

/// <summary>
/// 取得某月的第一天
/// </summary>
/// <param name="datetime">要取得月份第一天的时间</param>
/// <returns></returns>
public static DateTime FirstDayOfMonth(DateTime datetime)
{
    return datetime.AddDays(1 - datetime.Day);
}

/// <summary>
/// 取得某月的最后一天
/// </summary>
/// <param name="datetime">要取得月份最后一天的时间</param>
/// <returns></returns>
public static DateTime LastDayOfMonth(DateTime datetime)
{
    return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);
}

/// <summary>
/// 取得学期第一天
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public static DateTime FirstDayOfTerm(DateTime datetime)
{
     if (datetime.Month >= 2 && datetime.Month <= 7)
   {
     return new DateTime(datetime.Year, 2, 1);
   }
    else if (datetime.Month < 2)
   {
     return new DateTime(datetime.Year - 1, 8, 1);
   }
   else
  {
    return new DateTime(datetime.Year, 8, 1);
  }
}

/// <summary>
/// 取得学期最后一天
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public static DateTime LastDayOfTerm(DateTime datetime)
{
   return FirstDayOfTerm(datetime).AddMonths(7).AddDays(-1);
}

/// <summary>
/// 上周的第一天
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public static DateTime LastWeekFirstDay(DateTime datetime)
{
    int i = Convert.ToInt16(datetime.DayOfWeek);
    if (i == 0) { i = 7; }
    return datetime.AddDays(Convert.ToDouble((0 - i)) - 6);
}
/// <summary>
/// 上周的最后一天
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public static DateTime LastWeekEndDay(DateTime datetime)
{
    int i = Convert.ToInt16(datetime.DayOfWeek);
    if (i == 0) { i = 7; }
   return datetime.AddDays(Convert.ToDouble((6 - i)) - 6); //
}
/// <summary>
/// 上上周第一天
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public static DateTime TheWeekBeforeLastFirstDay(DateTime datetime)
{
    int i = Convert.ToInt16(datetime.DayOfWeek);
    if (i==0) { i = 7;}
    return datetime.AddDays(Convert.ToDouble((0-i))-13);
}
/// <summary>
/// 上上周最后一天
/// </summary>
/// <param name="datetime"></param>
/// <returns></returns>
public static DateTime TheWeekBeforeLastEndDay(DateTime datetime)
{
    int i = Convert.ToInt16(datetime.DayOfWeek);
    if (i == 0) { i = 7; }
    return datetime.AddDays(Convert.ToDouble((6-i)) - 15);
}
/// <summary>
/// 当前月的总天数
/// </summary>
/// <returns></returns>
public static int GetNowMonthCountDays()
{
    DateTime dtNow = DateTime.Now;
    int days = DateTime.DaysInMonth(dtNow.Year ,dtNow.Month);
    return days;
}