这并不像听起来那么简单,因为:
1)夏令时和标准时间之间的切换日期对于所有国家来说都不相同。
2)对于同一国家的所有年份,夏令时和标准时间之间的切换日期算法也不相同(例如,在中欧,以前是四月第一个星期日,现在是三月最后一个星期日)。美国从2007年开始,将切换日期从四月第一个星期日改为三月第二个星期日。
因此,仅仅提供一个简单的日期是不够的,你还需要知道地理位置。
但是,如果你可以接受这样一个事实,即你仅限于使用当前年份、当前地区(国家)的当前算法计算出的切换日期,并且这可能会在未来和过去的日期上出错,那么你可以使用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;
在我的电脑上(中欧时区),循环遍历2000年至2020年间的日期,结果如下:
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
但至少有些年份是错误的,因为算法已经在列出的年份中从我的语言环境改变了。
那么您的函数将如下:
FUNCTION IsDaylightSavingTime(Input : TDateTime) : BOOLEAN;
VAR
Start,Stop : TDateTime;
BEGIN
Result:=GetDaylightSavingsSwitchOverDates(YearOf(Input),Start,Stop) AND (Input>=Start) AND (Input<Stop)
END;