How to tackle daylight savings using TimeZone in J

问题:

I have to print the EST time in my Java application. I had set the time zone to EST using:

Calendar cal = Calendar.getInstance(TimeZone.getTimeZone(\"EST\"));

But when the daylight savings is being followed in this timezone, my code does not print the correct time (it prints 1 hour less).

How to make the code work to read the correct time always, irrespective of whether the daylight savings are being observed or not?

PS: I tried setting the timezone to EDT, but it doesn\'t solve the problem.

回答1:

This is the problem to start with:

Calendar cal = Calendar.getInstance(TimeZone.getTimeZone(\"EST\"));

The 3-letter abbreviations should be wholeheartedly avoided in favour of TZDB zone IDs. EST is Eastern Standard Time - and Standard time never observes DST; it\'s not really a full time zone name. It\'s the name used for part of a time zone. (Unfortunately I haven\'t come across a good term for this \"half time zone\" concept.)

You want a full time zone name. For example, America/New_York is in the Eastern time zone:

TimeZone zone = TimeZone.getTimeZone(\"America/New_York\");
DateFormat format = DateFormat.getDateTimeInstance();
format.setTimeZone(zone);

System.out.println(format.format(new Date()));

回答2:

Other answers are correct, especially the one by Jon Skeet, but outdated.

java.time

These old date-time classes have been supplanted by the java.time framework built into Java 8 and later.

If you simply want the current time in UTC, use the Instant class.

Instant now = Instant.now();

EST is not a time zone, as explained in the correct Answer by Jon Skeet. Such 3-4 letter codes are neither standardized nor unique, and further the confusion over Daylight Saving Time (DST). Use a proper time zone name in the \"continent/region\" format.

Perhaps you meant Eastern Standard Time in east coast of north America? Or Egypt Standard Time? Or European Standard Time?

ZoneId zoneId = ZoneId.of( \"America/New_York\" );
ZoneId zoneId = ZoneId.of( \"Africa/Cairo\" );
ZoneId zoneId = ZoneId.of( \"Europe/Lisbon\" );

Use any such ZoneId object to get the current moment adjusted to a particular time zone to produce a ZonedDateTime object.

ZonedDateTime zdt = ZonedDateTime.now( zoneId ) ;

Adjust that ZonedDateTime into a different time zone by producing another ZonedDateTime object from the first. The java.time framework uses immutable objects rather than changing (mutating) existing objects.

ZonedDateTime zdtGuam = zdt.withZoneSameInstant( ZoneId.of( \"Pacific/Guam\" ) ) ;

回答3:

Instead of entering \"EST\" for the timezone you can enter \"EST5EDT\" as such. As you noted, just \"EDT\" does not work. This will account for the daylight savings time issue. The code line looks like this:

Calendar cal = Calendar.getInstance(TimeZone.getTimeZone(\"EST5EDT\"));

回答4:

As per this answer:

TimeZone tz = TimeZone.getTimeZone(\"EST\");
boolean inDs = tz.inDaylightTime(new Date());

回答5:

public static float calculateTimeZone(String deviceTimeZone) {
    float ONE_HOUR_MILLIS = 60 * 60 * 1000;

    // Current timezone and date
    TimeZone timeZone = TimeZone.getTimeZone(deviceTimeZone);
    Date nowDate = new Date();
    float offsetFromUtc = timeZone.getOffset(nowDate.getTime()) / ONE_HOUR_MILLIS;

    // Daylight Saving time
    if (timeZone.useDaylightTime()) {
        // DST is used
        // I\'m saving this is preferences for later use

        // save the offset value to use it later
        float dstOffset = timeZone.getDSTSavings() / ONE_HOUR_MILLIS;
        // DstOffsetValue = dstOffset
        // I\'m saving this is preferences for later use
        // save that now we are in DST mode
        if (timeZone.inDaylightTime(nowDate)) {
            Log.e(Utility.class.getName(), \"in Daylight Time\");
            return -(ONE_HOUR_MILLIS * dstOffset);
        } else {
            Log.e(Utility.class.getName(), \"not in Daylight Time\");
            return 0;
        }
    } else
        return 0;
}

回答6:

private static Long DateTimeNowTicks(){
    long TICKS_AT_EPOCH = 621355968000000000L;
    TimeZone timeZone = Calendar.getInstance().getTimeZone();
    int offs = timeZone.getRawOffset();
    if (timeZone.inDaylightTime(new Date()))
        offs += 60 * 60 * 1000;
    return (System.currentTimeMillis() + offs) * 10000 + TICKS_AT_EPOCH;
}

回答7:

In java, DateFormatter by default uses DST,To avoid day Light saving (DST) you need to manually do a trick,
first you have to get the DST offset i.e. for how many millisecond DST applied, for ex somewhere DST is also for 45 minutes and for some places it is for 30 min
but in most cases DST is of 1 hour
you have to use Timezone object and check with the date whether it is falling under DST or not and then you have to manually add offset of DST into it. for eg:

 TimeZone tz = TimeZone.getTimeZone(\"EST\");
 boolean isDST = tz.inDaylightTime(yourDateObj);
 if(isDST){
 int sec= tz.getDSTSavings()/1000;// for no. of seconds
 Calendar cal= Calendar.getInstance();
 cal.setTime(yourDateObj);
 cal.add(Calendar.Seconds,sec);
 System.out.println(cal.getTime());// your Date with DST neglected
  }

回答8:

Implementing the TimeZone class to set the timezone to the Calendar takes care of the daylight savings.

java.util.TimeZone represents a time zone offset, and also figures out daylight savings.

sample code:

TimeZone est_timeZone = TimeZoneIDProvider.getTimeZoneID(TimeZoneID.US_EASTERN).getTimeZone();
Calendar enteredCalendar = Calendar.getInstance();
enteredCalendar.setTimeZone(est_timeZone);