注：一下方法的參數都為本地系統時間，即:datetime.now

/// <summary>
/// 計算本周起始日期（禮拜一的日期）
/// </summary>
/// <param name="someDate">該周中任意一天</param>
/// <returns>返回禮拜一日期，后面的具體時、分、秒和傳入值相等</returns>
public static DateTime CalculateFirstDateOfWeek(DateTime someDate)
{
int i = someDate.DayOfWeek - DayOfWeek.Monday;
if (i == -1) i = 6;// i值 > = 0 ，因為枚舉原因，Sunday排在最前，此時Sunday-Monday=-1，必須+7=6。
TimeSpan ts = new TimeSpan(i, 0, 0, 0);
return someDate.Subtract(ts).Date;
}

/// <summary>
/// 計算本周結束日期（禮拜五的日期）
/// </summary>
/// <param name="someDate">該周中任意一天</param>
/// <returns>返回禮拜日日期，后面的具體時、分、秒和傳入值相等</returns>
public static DateTime CalculateLastDateOfWeek(DateTime someDate)
{
int i = someDate.DayOfWeek - DayOfWeek.Sunday;
if (i != 0) i = 7 - i;// 因為枚舉原因，Sunday排在最前，相減間隔要被7減。
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>
/// 判斷選擇的日期是否是本周（根據系統當前時間決定的‘本周'比較而言）
/// </summary>
/// <param name="someDate"></param>
/// <returns></returns>
public static bool IsThisWeek(DateTime someDate)
{
//得到someDate對應的周一
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時間</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時間</param>
/// <returns></returns>
public static DateTime LastDayOfMonth(DateTime datetime)
{
return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);
}

/// <summary>
/// 取得學期第一天
/// </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>
/// 取得學期最后一天
/// </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>
/// 當前月的總天數
/// </summary>
/// <returns></returns>
public static int GetNowMonthCountDays()
{
DateTime dtNow = DateTime.Now;
int days = DateTime.DaysInMonth(dtNow.Year ,dtNow.Month);
return days;
}

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