考慮夏時制時間將本地時間轉換爲UTC時間或反之亦然

Question

我知道如何將本地時間轉換爲UTC時間，反之亦然。 但我在做這件事的時候非常困惑夏令時（DST）的處理。

所以任何人都可以回答下面的問題：
1.當在時區之間轉換時，java是否內部處理DST？
2.在時區之間轉換時我需要做什麼？
3.什麼好的文章更清楚地解釋這一點？

在此先感謝。

Answer 1

您確定您知道如何將日期轉換爲UTC並返回？正確？
恐怕我懷疑。

1. 是的。
2. 你不需要轉換，你只需要分配正確的TimeZone。
3. 您需要什麼文章？好的，我正在研究這個問題，但現在讓我在這裏回答一個問題。

首先是第一件事。您的程序應在內部以UTC時區存儲日期（或日曆）。那麼，事實上在格林尼治標準時間，因爲在Java中沒有閏秒，但這是另一回事。
當你需要「轉換」時，唯一的地方是當你要顯示給用戶的時間。這也涉及發送電子郵件。在這兩種情況下，您需要格式日期以獲取其文本表示形式。爲了您將使用DateFormat並指定正確的時區：

// that's for desktop application 
    // for web application one needs to detect Locale 
    Locale locale = Locale.getDefault(); 
    // again, this one works for desktop application 
    // for web application it is more complicated 
    TimeZone currentTimeZone = TimeZone.getDefault(); 
    // in fact I could skip this line and get just DateTime instance, 
    // but I wanted to show how to do that correctly for 
    // any time zone and locale 
    DateFormat formatter = DateFormat.getDateTimeInstance(
      DateFormat.DEFAULT, 
      DateFormat.DEFAULT, 
      locale); 
    formatter.setTimeZone(currentTimeZone); 

    // Dates "conversion" 
    Date currentDate = new Date(); 
    long sixMonths = 180L * 24 * 3600 * 1000; 
    Date inSixMonths = new Date(currentDate.getTime() + sixMonths); 

    System.out.println(formatter.format(currentDate)); 
    System.out.println(formatter.format(inSixMonths)); 
    // for me it prints 
    // 2011-05-14 16:11:29 
    // 2011-11-10 15:11:29 

    // now for "UTC" 
    formatter.setTimeZone(TimeZone.getTimeZone("UTC")); 
    System.out.println(formatter.format(currentDate)); 
    System.out.println(formatter.format(inSixMonths)); 
    // 2011-05-14 14:13:50 
    // 2011-11-10 14:13:50 

正如你所看到的，Java的關心處理DST。你當然可以手動處理它，只需閱讀TimeZone related JavaDoc。

Answer 2

這是我找到的最佳解決方案。我在這裏複製它，但解決方案來自http://biese.wordpress.com/2014/02/28/the-easy-way-to-convert-local-time-to-utc-time/。

package com.test.timezone; 

import java.util.TimeZone; 

public final class Utility { 
    public static final TimeZone utcTZ = TimeZone.getTimeZone("UTC"); 

    public static long toLocalTime(long time, TimeZone to) { 
     return convertTime(time, utcTZ, to); 
    } 

    public static long toUTC(long time, TimeZone from) { 
     return convertTime(time, from, utcTZ); 
    } 

    public static long convertTime(long time, TimeZone from, TimeZone to) { 
     return time + getTimeZoneOffset(time, from, to); 
    } 

    private static long getTimeZoneOffset(long time, TimeZone from, TimeZone to) { 
     int fromOffset = from.getOffset(time); 
     int toOffset = to.getOffset(time); 
     int diff = 0; 

     if (fromOffset >= 0){ 
      if (toOffset > 0){ 
       toOffset = -1*toOffset; 
      } else { 
       toOffset = Math.abs(toOffset); 
      } 
      diff = (fromOffset+toOffset)*-1; 
     } else { 
      if (toOffset <= 0){ 
       toOffset = -1*Math.abs(toOffset); 
      } 
      diff = (Math.abs(fromOffset)+toOffset); 
     } 
     return diff; 
    } 
} 

package com.test.timezone; 

import java.text.SimpleDateFormat; 
import java.util.Calendar; 
import java.util.GregorianCalendar; 
import java.util.TimeZone; 

public class TestTimezone { 

    public static void main(String[] args) { 
     SimpleDateFormat sdf = new SimpleDateFormat("yyyy MMM dd HH:mm:ss zzzz"); 
     Calendar date1 = new GregorianCalendar(2014,0,15,10,0,0); 
     System.out.println(sdf.format(date1.getTime())+"\n"); 
     long utcTimeStamp = Utility.toUTC(date1.getTimeInMillis(), date1.getTimeZone()); 
     Calendar utcCal = Calendar.getInstance(); 
     utcCal.setTimeInMillis(utcTimeStamp); 
     System.out.println("toUTC: "+sdf.format(utcCal.getTime())+"\n"); 

     System.out.println("---------------------------------------"); 
     Calendar date2 = new GregorianCalendar(2014,2,15,10,0,0); 
     System.out.println(sdf.format(date2.getTime())+"\n"); 
     utcTimeStamp = Utility.toUTC(date2.getTimeInMillis(), date2.getTimeZone()); 
     utcCal.setTimeInMillis(utcTimeStamp); 
     System.out.println("toUTC: "+sdf.format(utcCal.getTime())+"\n"); 

     System.out.println("---------------------------------------"); 
     Calendar date3 = new GregorianCalendar(2014,11,25,9,0,0); 
     System.out.println(sdf.format(date3.getTime())+"\n"); 
     long uTime = Utility.toUTC(date3.getTimeInMillis(), date3.getTimeZone()); 
     System.out.println("utcTimeStamp: "+uTime+"\n"); 
     long lTime = Utility.toLocalTime(uTime, TimeZone.getTimeZone("EST")); 
     Calendar locCal = Calendar.getInstance(); 
     locCal.setTimeInMillis(lTime); 
     System.out.println("toLocal: "+sdf.format(locCal.getTime())+"\n"); 

     System.out.println("---------------------------------------"); 
     Calendar date4 = new GregorianCalendar(2014,6,4,9,0,0); 
     System.out.println(sdf.format(date4.getTime())+"\n"); 
     uTime = Utility.toUTC(date4.getTimeInMillis(), date4.getTimeZone()); 
     System.out.println("utcTimeStamp: "+uTime+"\n"); 
     lTime = Utility.toLocalTime(uTime, TimeZone.getTimeZone("EST")); 
     locCal = Calendar.getInstance(); 
     locCal.setTimeInMillis(lTime); 
     System.out.println("toLocal: "+sdf.format(locCal.getTime())+"\n"); 
    } 
} 

Answer 3

在TALE的答案的代碼可以簡化爲：

public final class Utility { 
    public static long toLocalTime(long time, TimeZone to) { 
     return time + to.getOffset(time); 
    } 

    public static long toUTC(long time, TimeZone from) { 
     return time - from.getOffset(time); 
    } 
}