java - 无法将 UTC 转换为本地时间

Question

我正在编写Java代码。我能够将本地时间转换为UTC。但是当我想将其转换回本地时间时，我遇到了问题。我正在执行的步骤-

1. 获取当前的毫秒数。(完成)
2. 将当前毫秒(millisSinceEpoch) 转换为当前本地时间。(DONE)
3. 将当前毫秒(millisSinceEpoch) 转换为 UTC 时间。(完成)
4. 将 UTC 时间转换为 UTC 毫秒。(完成)
5. 将 UTC 毫秒转换为 UTC 时间。(完成)

6. 将 UTC 毫秒转换为本地时间(IST)(这里面临问题.....在这里获取相同的 UTC 时间。为什么它不转换为本地时间？)。有一个答案，其中多个日期创建了对象，但这里我没有创建多个日期对象并且仅在毫秒上工作。我的代码-

   import java.text.DateFormat;
   import java.text.ParseException;
   import java.text.SimpleDateFormat;
   import java.time.Instant;
   import java.time.ZonedDateTime;
   import java.util.Date;
   import java.util.TimeZone;
   public class TimeApiClass {
   public static void main(String args[]) throws ParseException {
   ZonedDateTime currentZone = ZonedDateTime.now();
   
   //Getting milliseconds of the present
   Instant instant=currentZone.toInstant();
   long millisSinceEpoch=instant.toEpochMilli();
   System.out.println("Not Converted to UTC-(Milliseconds-in local)");
   System.out.println(millisSinceEpoch);                                //Output1->1579788244731
   
   //Converting present milliseconds(millisSinceEpoch) to present local time
   SimpleDateFormat sdf3 = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS ");
   String str3=sdf3.format(new Date(millisSinceEpoch));
   System.out.println("Local Time-Not Converted to UTC-(Present local time)");
   System.out.println(str3);                                           //2020/01/23 19:34:04.731
   
   
   
   //Converting present milliseconds(millisSinceEpoch) to UTC time
   SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS ");
   sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
   String str=sdf.format(new Date(millisSinceEpoch));
   System.out.println("UTC time-Local time Converted to UTC(unrequired time format)-");
   System.out.println(str);// str contains 2020/01/22 13:22:55 UTC     //Output3 ->2020/01/23 14:04:04.731
   
   System.out.println("-----------------------------------------------------------------------------------------");
   
   
   //Converting UTC time to UTC milliseconds
   String myDate = str;  // myDate and str contains 2020/01/22 10:08:42
   SimpleDateFormat sdff = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS ");
   Date date = sdff.parse(myDate);
   long millis = date.getTime();
   System.out.println("Converted to milliseconds(Now its in required format in UTC milliseconds)");
   System.out.println(millis);                                       //Output4 ->1579749867000
   
   
   //Convert UTC milliseconds to UTC Time
   DateFormat simple = new SimpleDateFormat(" yyyy/MM/dd HH:mm:ss.SSS");
   Date result=new Date(millis);
   System.out.println("Converting UTC milliseconds back to date/time format-");
   System.out.println(simple.format(result));
   
   
   //Convert UTC milliseconds to local time(IST)
   Date dateee=new Date(millis);         //Facing the problem here.....Getting the same UTC time here.Why is it not converting to Local time?
   DateFormat format=new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS");
   format.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));
   System.out.println("lllllllllllllllllllllllll");
   System.out.println(format.format(dateee));
   
   
   
   
   //Converting UTC time to IST time
   SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSS ");
   sdf2.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));
   String istTime=sdf2.format(new Date(millis));
   System.out.println("Now converting UTC millis to IST format->");
   System.out.println(istTime);                                     //Output5 ->2020/01/23 08:54:27 IST
   
   
   //Converting IST time to milliseconds,here I am facing the problem.My output has to give the local milliseconds as I am passing ISTtime here,but its giving me UTC milliseconds,Which I dont want.
   Date date66=sdf2.parse(istTime);
   long finalMillis=date66.getTime();
   System.out.println("Now converting IST to IST milliseconds:");
   System.out.println(finalMillis);                               //Output6 ->1579749867000
   

   } } //1579613838087

Answer 1

tl;博士

通过 ZonedDateTime.withZoneSameInstant(ZoneId) 将 ZonedDateTime 转换为不同的时区

<小时/>

不建议您使用 java.util.SimpleDateFormat 进行日期和时间操作，并且您的代码显示部分使用 java.time，这意味着您可以只需完全切换到 java.time 即可。对于格式化，请使用java.time.format.DateTimeFormatter。

请参阅此示例:

public static void main(String[] args) {
    // get the current instant
    Instant instant = Instant.now();
    // and convert it to a datetime object representing date and time in UTC
    ZonedDateTime ofInstant = ZonedDateTime.ofInstant(instant, ZoneId.of("UTC"));
    // print it
    System.out.println("Now from Instant in UTC:\t"
            + ofInstant.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));

    // you can have a shorter way: current moment in your system's time zone
    ZonedDateTime nowHere = ZonedDateTime.now();
    // or in UTC
    ZonedDateTime utcNow = ZonedDateTime.now(ZoneId.of("UTC"));

    // print them both
    System.out.println("Now in my (system's) time zone:\t"
            + nowHere.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
    System.out.println("Now in UTC:\t\t\t"
            + utcNow.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));

    // then format the UTC datetime back to the one your system has
    ZonedDateTime backFromUtc = utcNow.withZoneSameInstant(ZoneId.systemDefault());
    // and print that one, too
    System.out.println("Back from UTC:\t\t\t"
            + backFromUtc.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
}

输出类似于:

Now from Instant in UTC:        2020-01-23T15:44:11.921Z[UTC]
Now in my (system's) time zone: 2020-01-23T16:44:12.091+01:00[Europe/Berlin]
Now in UTC:                     2020-01-23T15:44:12.091Z[UTC]
Back from UTC:                  2020-01-23T16:50:52.194+01:00[Europe/Berlin]