Delphi – 相当于需要C#DateTime.IsDaylightSavingTime()方法

1)DST和标准时间之间的切换日期对于所有国家都不相同

2)DST和标准时间之间的转换日期与所有年份的同一国家的算法不同(例如,在中欧,它之前是4月的第一个星期日,IIRC,现在是3月的最后一个星期日).美国从4月的第一个星期日变为2007年3月的第二个星期日.

所以 – 一个简单的日期是不够的,你还需要一个地理位置.

但是,如果您能够接受这样的事实,那么您可以将自己限制为可以根据CURRENT语言环境(国家/地区)的CURRENT年的CURRENT算法计算的切换日期,并且这可能对于日期中的日期都是错误的.未来和过去,您可以使用TIME_ZONE_INFORMATION中的信息来计算切换日期：

USES Windows,SysUtils,DateUtils;

FUNCTION GetDaylightSavingsSwitchOverDates(Year : Cardinal ; VAR Start,Stop : TDateTime) : BOOLEAN;

  VAR
    TZ : TTimeZoneInformation;

  FUNCTION DecodeSwitchOverDate(Year : Cardinal ; CONST Time : TSystemTime) : TDateTime;
    VAR
      I : Cardinal;

    BEGIN
      Result:=EncodeDateTime(Year,Time.wMonth,1,Time.wHour,Time.wMinute,Time.wSecond,0);
      IF Time.wDay=5 THEN BEGIN
        Result:=DateOf(EndOfTheMonth(Result))+TimeOf(Result);
        WHILE PRED(DayOfWeek(Result))<>Time.wDayOfWeek DO
          Result:=IncDay(Result,-1)
        END
      ELSE BEGIN
        WHILE PRED(DayOfWeek(Result))<>Time.wDayOfWeek DO Result:=IncDay(Result);
        FOR I:=1 TO PRED(Time.wDay) DO Result:=IncWeek(Result)
      END
    END;

  BEGIN
    IF GetTimeZoneInformation(TZ)=TIME_ZONE_ID_UNKNOWN THEN
      Result:=FALSE
    ELSE BEGIN
      Start:=DecodeSwitchOverDate(Year,TZ.DaylightDate);
      Stop:=DecodeSwitchOverDate(Year,TZ.StandardDate);
      Result:=TRUE
    END
  END;

FUNCTION StartOfDST(Year : Cardinal) : TDateTime;
  VAR
    Stop : TDateTime;

  BEGIN
    IF NOT GetDaylightSavingsSwitchOverDates(Year,Result,Stop) THEN Result:=0
  END;

FUNCTION EndOfDST(Year : Cardinal) : TDateTime;
  VAR
    Start : TDateTime;

  BEGIN
    IF NOT GetDaylightSavingsSwitchOverDates(Year,Start,Result) THEN Result:=0
  END;

在我的PC(中欧时区)上循环使用2000年到2020年,我得到以下日期：

DST in 2000: Sun 26 Mar 2000 02:00:00 through Sun 29 Oct 2000 03:00:00
DST in 2001: Sun 25 Mar 2001 02:00:00 through Sun 28 Oct 2001 03:00:00
DST in 2002: Sun 31 Mar 2002 02:00:00 through Sun 27 Oct 2002 03:00:00
DST in 2003: Sun 30 Mar 2003 02:00:00 through Sun 26 Oct 2003 03:00:00
DST in 2004: Sun 28 Mar 2004 02:00:00 through Sun 31 Oct 2004 03:00:00
DST in 2005: Sun 27 Mar 2005 02:00:00 through Sun 30 Oct 2005 03:00:00
DST in 2006: Sun 26 Mar 2006 02:00:00 through Sun 29 Oct 2006 03:00:00
DST in 2007: Sun 25 Mar 2007 02:00:00 through Sun 28 Oct 2007 03:00:00
DST in 2008: Sun 30 Mar 2008 02:00:00 through Sun 26 Oct 2008 03:00:00
DST in 2009: Sun 29 Mar 2009 02:00:00 through Sun 25 Oct 2009 03:00:00
DST in 2010: Sun 28 Mar 2010 02:00:00 through Sun 31 Oct 2010 03:00:00
DST in 2011: Sun 27 Mar 2011 02:00:00 through Sun 30 Oct 2011 03:00:00
DST in 2012: Sun 25 Mar 2012 02:00:00 through Sun 28 Oct 2012 03:00:00
DST in 2013: Sun 31 Mar 2013 02:00:00 through Sun 27 Oct 2013 03:00:00
DST in 2014: Sun 30 Mar 2014 02:00:00 through Sun 26 Oct 2014 03:00:00
DST in 2015: Sun 29 Mar 2015 02:00:00 through Sun 25 Oct 2015 03:00:00
DST in 2016: Sun 27 Mar 2016 02:00:00 through Sun 30 Oct 2016 03:00:00
DST in 2017: Sun 26 Mar 2017 02:00:00 through Sun 29 Oct 2017 03:00:00
DST in 2018: Sun 25 Mar 2018 02:00:00 through Sun 28 Oct 2018 03:00:00
DST in 2019: Sun 31 Mar 2019 02:00:00 through Sun 27 Oct 2019 03:00:00
DST in 2020: Sun 29 Mar 2020 02:00:00 through Sun 25 Oct 2020 03:00:00

但由于算法在列出的年份中从我的语言环境发生了变化,因此这些年中至少有一些是不正确的.

那么你的功能是：

FUNCTION IsDaylightSavingTime(Input : TDateTime) : BOOLEAN;
  VAR
    Start,Stop : TDateTime;

  BEGIN
    Result:=GetDaylightSavingsSwitchOverDates(YearOf(Input),Stop) AND (Input>=Start) AND (Input<Stop)
  END;