考虑夏令时，将本地时间转换为 UTC 时间，反之亦然

Question

我知道如何将本地时间转换为UTC时间，反之亦然。但是在执行此操作时，我对夏令时（DST）处理感到非常困惑。

所以任何人都可以回答以下问题：
1. 在时区之间转换时，java 是否在内部处理 DST？
2. 在时区之间转换时我需要做什么？
3. 有什么好文章可以更清楚地解释这一点吗？

提前谢谢。

Answer 1

您确定知道如何将日期转换为 UTC 并返回吗？正确？
恐怕，我对此表示怀疑。

1. 是的。
2. 您不需要转换，只需要分配正确的时区。
3. 你需要一篇文章做什么？好的，我正在努力解决这个问题，但现在让我在这里给出一个答案。

首先是。程序应在内部以 UTC 时区存储日期（或日历）。好吧，事实上在格林威治标准时间，因为Java中没有闰秒，但那是另一回事了。
唯一需要"转换"的地方是要向用户显示时间的时间。这也与发送电子邮件有关。在这两种情况下，您都需要格式化日期以获取其文本表示形式。为此，您将使用DateFormat并分配正确的时区：

    // 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

如您所见，Java关心处理DST。您当然可以手动处理它，只需阅读与时区相关的JavaDoc即可。

Answer 2

这是我找到的最佳解决方案。 我在这里复制它，但解决方案来自 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

TALE答案中的代码可以简化：

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);
    }
}