首先，我想宣告我知道 Java Calendar 類正在被其他可以說更好的庫所取代。也許我偶然發現了 Calendar 失寵的原因之一。
我在日歷中遇到了令人沮喪的行為，因為它涉及夏令時結束時的重疊小時。
public void annoying_issue()
{
Calendar midnightPDT = Calendar.getInstance(TimeZone.getTimeZone("US/Pacific"));
midnightPDT.set(Calendar.YEAR, 2021);
midnightPDT.set(Calendar.MONTH, 10);
midnightPDT.set(Calendar.DAY_OF_MONTH, 7);
midnightPDT.set(Calendar.HOUR_OF_DAY, 0);
midnightPDT.set(Calendar.MINUTE, 0);
midnightPDT.set(Calendar.SECOND, 0);
midnightPDT.set(Calendar.MILLISECOND, 0);
Calendar oneAMPDT = Calendar.getInstance(TimeZone.getTimeZone("US/Pacific"));
oneAMPDT.setTimeInMillis(midnightPDT.getTimeInMillis() (60*60*1000));//this is the easiest way I've found to get to the first 1am hour at DST overlap
System.out.println(new Date(midnightPDT.getTimeInMillis()));//prints the expected "Sun Nov 7 00:00:00 PDT 2021"
System.out.println(new Date(oneAMPDT.getTimeInMillis()));//prints "Sun Nov 7 01:00:00 PDT 2021" also expected
oneAMPDT.clear(Calendar.MINUTE);//minute is already 0 so no change should occur... RIGHT!?
//WRONG!!!!
//The time is now in PST! The millisecond value has increased by 3600000, too!!
System.out.println(new Date(oneAMPDT.getTimeInMillis()));//prints "Sun Nov 7 01:00:00 PST 2021"
}
跟隨評論，您將看到清除日歷中的 MINUTE 欄位實際上將其向上移動了一個小時！見鬼！？
這也發生在我使用 oneAMPDT.set(Calendar.MINUTE, 0)
這是預期的行為嗎？有沒有辦法防止這種情況？
uj5u.com熱心網友回復：
避免遺留的日期時間類；如果需要轉換
正如您所指出的，Calendar多年前被JSR 310（一致采用）中定義的java.time類所取代。正如您所注意到的，有很多原因可以避免使用Calendar&Date等。
如果您必須有一個Calendar物件與尚未更新為java.time 的舊代碼進行互操作，請在java.time 中完成您的作業后進行轉換。
時間
指定所需的時區。請注意，這US/Pacific只是實際時區的別名，America/Los_Angeles.
ZoneId zLosAngeles = ZoneId.of( "America/Los_Angeles" ) ;
指定您想要的時刻。
LocalDate ld = LocalDate.of( 2021 , Month.NOVEMBER , 7 ) ;
在您的代碼中，您似乎假設一天中的第一個時刻發生在 00:00。情況并非總是如此。某些時區中的某些日期可能會在其他時間開始。所以讓java.time確定一天中的第一個時刻。
ZonedDateTime firstMomentOfThe7thInLosAngeles = ld.atStartOfDay( zLosAngeles ) ;
firstMomentOfThe7thInLosAngeles.toString(): 2021-11-07T00:00-07:00[America/Los_Angeles]
但隨后你又跳到了另一個時刻，凌晨 1 點。
ZonedDateTime oneAmOnThe7thLosAngeles = firstMomentOfThe7thInLosAngeles.with( LocalTime.of( 1 , 0 ) ) ;
oneAmOnThe7thLosAngeles.toString(): 2021-11-07T01:00-07:00[America/Los_Angeles]
該時間在該區域中的該日期可能存在也可能不存在。ZonedDateTime如果需要，班級將進行調整。
您使用midnightPDT了變數的名稱。我建議避免使用該術語，midnight因為它的使用會在沒有精確定義的情況下混淆日期時間處理。如果這就是您的意思，我建議使用術語“一天中的第一時刻”。
您提取自 1970 年第一時刻的紀元參考以來的毫秒數，如 UTC，1970-01-01T00:00Z。
Instant firstMomentOfThe7thInLosAngelesAsSeenInUtc = firstMomentOfThe7thInLosAngeles.toInstant() ;
long millisSinceEpoch_FirstMomentOf7thLosAngeles = firstMomentOfThe7thInLosAngelesAsSeenInUtc.toEpochMilli() ;
firstMomentOfThe7thInLosAngelesAsSeenInUtc.toString(): 2021-11-07T07:00:00Z
millisSinceEpoch_FirstMomentOf7thLosAngeles = 1636268400000
對于我們凌晨 1 點的時刻，您也這樣做。
Instant oneAmOnThe7thLosAngelesAsSeenInUtc = oneAmOnThe7thLosAngeles.toInstant() ;
long millisSinceEpoch_OneAmOn7thLosAngeles = oneAmOnThe7thLosAngelesAsSeenInUtc.toEpochMilli() ;
oneAmOnThe7thLosAngelesAsSeenInUtc.toString(): 2021-11-07T08:00:00Z
millisSinceEpoch_OneAmOn7thLosAngeles = 1636272000000
我們應該看到一小時的差異。一小時 = 3,600,000 = 60 * 60 * 1,000。
long diff = ( millisSinceEpoch_OneAmOn7thLosAngeles - millisSinceEpoch_FirstMomentOf7thLosAngeles ); // 3,600,000 = 60 * 60 * 1,000.
差異 = 3600000
切換
然后你繼續提到夏令時 (DST)轉換。當天美國 DST 的轉換時間是凌晨 2 點，而不是凌晨 1 點。在凌晨 2 點到達的那一刻，時鐘又回到了凌晨 1 點，又是一個 1:00-2:00 小時。
為了達到那個切換點，讓我們增加一個小時。
ZonedDateTime cutover_Addition = oneAmOnThe7thLosAngeles.plusHours( 1 );
cutover_Addition = 2021-11-07T01:00-08:00[America/Los_Angeles]
Notice that the time-of-day shows the same (1 AM), but the offset-from-UTC has changed from being 7 hours behind UTC to now 8 hours behind UTC. There lies the hour difference you seek.
Let's get the count of milliseconds since epoch for this third moment. Before we had first moment of the day (00:00), then the first occurring 1 AM, and now we have the second occurring 1 AM on this “Fall-Back” date of November 7, 2021.
long millisSinceEpoch_Cutover = cutover_Addition.toInstant().toEpochMilli();
1636275600000
Duration.between( firstMomentOfThe7thInLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) = PT2H
Duration.between( oneAmOnThe7thLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) = PT1H
The ZonedDateTime class does offer a pair of methods of use at these moments of cutover: withEarlierOffsetAtOverlap and withLaterOffsetAtOverlap.
ZonedDateTime cutover_OverlapEarlier =
cutover_Addition
.withEarlierOffsetAtOverlap();
ZonedDateTime cutover_OverlapLater =
cutover_Addition
.withLaterOffsetAtOverlap();
cutover_OverlapEarlier = 2021-11-07T01:00-07:00[美國/洛杉磯]
cutover_OverlapLater = 2021-11-07T01:00-08:00[美國/洛杉磯]
Calendar
如果您確實需要一個Calendar物件，只需轉換即可。
Calendar x = GregorianCalendar.from( firstMomentOfThe7thInLosAngeles ) ;
Calendar y = GregorianCalendar.from( oneAmOnThe7thLosAngeles ) ;
Calendar z = GregorianCalendar.from( cutover_Addition );
如果你的目標只是努力理解Calendar階級行為，我建議你停止受虐狂。無關緊要。Sun、Oracle 和 JCP 社區都放棄了那些糟糕的遺留日期時間類。我建議你這樣做。
示例代碼
將上面的所有代碼放在一起。
ZoneId zLosAngeles = ZoneId.of( "America/Los_Angeles" );
LocalDate ld = LocalDate.of( 2021 , Month.NOVEMBER , 7 );
ZonedDateTime firstMomentOfThe7thInLosAngeles = ld.atStartOfDay( zLosAngeles );
ZonedDateTime oneAmOnThe7thLosAngeles = firstMomentOfThe7thInLosAngeles.with( LocalTime.of( 1 , 0 ) );
Instant firstMomentOfThe7thInLosAngelesAsSeenInUtc = firstMomentOfThe7thInLosAngeles.toInstant();
long millisSinceEpoch_FirstMomentOf7thLosAngeles = firstMomentOfThe7thInLosAngelesAsSeenInUtc.toEpochMilli();
Instant oneAmOnThe7thLosAngelesAsSeenInUtc = oneAmOnThe7thLosAngeles.toInstant();
long millisSinceEpoch_OneAmOn7thLosAngeles = oneAmOnThe7thLosAngelesAsSeenInUtc.toEpochMilli();
long diff = ( millisSinceEpoch_OneAmOn7thLosAngeles - millisSinceEpoch_FirstMomentOf7thLosAngeles ); // 3,600,000 = 60 * 60 * 1,000.
ZonedDateTime cutover_Addition = oneAmOnThe7thLosAngeles.plusHours( 1 );
long millisSinceEpoch_Cutover = cutover_Addition.toInstant().toEpochMilli();
ZonedDateTime cutover_OverlapEarlier =
cutover_Addition
.withEarlierOffsetAtOverlap();
ZonedDateTime cutover_OverlapLater =
cutover_Addition
.withLaterOffsetAtOverlap();
如果需要，轉換為遺留類。
Calendar x = GregorianCalendar.from( firstMomentOfThe7thInLosAngeles );
Calendar y = GregorianCalendar.from( oneAmOnThe7thLosAngeles );
Calendar z = GregorianCalendar.from( cutover_Addition );
轉儲到控制臺。
System.out.println( "firstMomentOfThe7thInLosAngeles = " firstMomentOfThe7thInLosAngeles );
System.out.println( "oneAmOnThe7thLosAngeles = " oneAmOnThe7thLosAngeles );
System.out.println( "firstMomentOfThe7thInLosAngelesAsSeenInUtc = " firstMomentOfThe7thInLosAngelesAsSeenInUtc );
System.out.println( "millisSinceEpoch_FirstMomentOf7thLosAngeles = " millisSinceEpoch_FirstMomentOf7thLosAngeles );
System.out.println( "oneAmOnThe7thLosAngelesAsSeenInUtc = " oneAmOnThe7thLosAngelesAsSeenInUtc );
System.out.println( "millisSinceEpoch_OneAmOn7thLosAngeles = " millisSinceEpoch_OneAmOn7thLosAngeles );
System.out.println( "diff = " diff );
System.out.println( "x = " x );
System.out.println( "y = " y );
System.out.println( "z = " z );
System.out.println( "cutover_Addition = " cutover_Addition );
System.out.println( "millisSinceEpoch_Cutover = " millisSinceEpoch_Cutover );
System.out.println( "Duration.between( firstMomentOfThe7thInLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) = " Duration.between( firstMomentOfThe7thInLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) );
System.out.println( "Duration.between( oneAmOnThe7thLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) = " Duration.between( oneAmOnThe7thLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) );
System.out.println( "cutover_OverlapEarlier = " cutover_OverlapEarlier );
System.out.println( "cutover_OverlapLater = " cutover_OverlapLater );
跑的時候。
firstMomentOfThe7thInLosAngeles = 2021-11-07T00:00-07:00[America/Los_Angeles]
oneAmOnThe7thLosAngeles = 2021-11-07T01:00-07:00[America/Los_Angeles]
firstMomentOfThe7thInLosAngelesAsSeenInUtc = 2021-11-07T07:00:00Z
millisSinceEpoch_FirstMomentOf7thLosAngeles = 1636268400000
oneAmOnThe7thLosAngelesAsSeenInUtc = 2021-11-07T08:00:00Z
millisSinceEpoch_OneAmOn7thLosAngeles = 1636272000000
diff = 3600000
x = java.util.GregorianCalendar[time=1636268400000,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2021,MONTH=10,WEEK_OF_YEAR=44,WEEK_OF_MONTH=1,DAY_OF_MONTH=7,DAY_OF_YEAR=311,DAY_OF_WEEK=1,DAY_OF_WEEK_IN_MONTH=1,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-28800000,DST_OFFSET=3600000]
y = java.util.GregorianCalendar[time=1636272000000,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2021,MONTH=10,WEEK_OF_YEAR=44,WEEK_OF_MONTH=1,DAY_OF_MONTH=7,DAY_OF_YEAR=311,DAY_OF_WEEK=1,DAY_OF_WEEK_IN_MONTH=1,AM_PM=0,HOUR=1,HOUR_OF_DAY=1,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-28800000,DST_OFFSET=3600000]
z = java.util.GregorianCalendar[time=1636275600000,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/Los_Angeles",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=185,lastRule=java.util.SimpleTimeZone[id=America/Los_Angeles,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=2,minimalDaysInFirstWeek=4,ERA=1,YEAR=2021,MONTH=10,WEEK_OF_YEAR=44,WEEK_OF_MONTH=1,DAY_OF_MONTH=7,DAY_OF_YEAR=311,DAY_OF_WEEK=1,DAY_OF_WEEK_IN_MONTH=1,AM_PM=0,HOUR=1,HOUR_OF_DAY=1,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-28800000,DST_OFFSET=0]
cutover_Addition = 2021-11-07T01:00-08:00[America/Los_Angeles]
millisSinceEpoch_Cutover = 1636275600000
Duration.between( firstMomentOfThe7thInLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) = PT2H
Duration.between( oneAmOnThe7thLosAngelesAsSeenInUtc , cutover_Addition.toInstant() ) = PT1H
cutover_OverlapEarlier = 2021-11-07T01:00-07:00[America/Los_Angeles]
cutover_OverlapLater = 2021-11-07T01:00-08:00[America/Los_Angeles]
uj5u.com熱心網友回復：
時間
這是預期的行為嗎？不，我認為這是一個錯誤。
有沒有辦法防止這種情況？是的，您已經提到或至少暗示的方式：使用ZonedDateTime而不是Calendar. Basil Bourque 已經說過了。作為一個適度的補充，我想展示從Calendar到的完整往返ZonedDateTime，將分鐘設定為 0 并轉換回Calendar. 如果您需要它與遺留代碼的互操作性。
GregorianCalendar oneAmPdt = new GregorianCalendar(TimeZone.getTimeZone(ZoneId.of("America/Los_Angeles")));
oneAmPdt.clear();
oneAmPdt.set(2021, Calendar.NOVEMBER, 7, 0, 0);
oneAmPdt.add(Calendar.HOUR_OF_DAY, 1);
System.out.println(oneAmPdt.getTime());
ZonedDateTime zdt = oneAmPdt.toZonedDateTime();
// Minute is already 0 so no change should occur... RIGHT!?
zdt = zdt.withMinute(0);
oneAmPdt = GregorianCalendar.from(zdt);
System.out.println(oneAmPdt.getTime());
輸出：
Sun Nov 07 01:00:00 PDT 2021 Sun Nov 07 01:00:00 PDT 2021
但是我用過GregorianCalendar，不是Calendar嗎？你也是。GregorianCalendar是Calendar您從Calendar.getIntance(). 在某些環境中，您會得到一個不同的子類，反映在那里使用的日歷系統，并且您最初呼叫 的set不會給您預期的結果。在這種情況下，您需要a GregorianCalendar（如果您ZonedDateTime從一開始就無法擁有 a ）。
在修改我們的舊代碼時，即使不是為了規避舊代碼Calendar或GregorianCalendar類中的錯誤，我也可能會以上述方式進行。這是向 java.time 的長期過渡的一小步。
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標籤：爪哇 约会时间 日历 日期 java.util.calendar
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