Java 中的时区缩写

Question

I have a collection of Java Date objects, loaded from an old database which did not store timezone info.

I know that all the dates (which include hours, minutes and seconds) are for the US Eastern time zone.

I want to derive the correct timezone abbreviation ("EST" or "EDT") for each date.

Here is my attempt (using Java 11). It is unwieldy, to say the least.

I looked into the following classes:

java.util.TimeZone
java.time.ZoneId
java.time.zone.ZoneRules

Here is my method - it uses two statements purely to give me a fighting chance of still understanding what I wrote, a week from now.

EDIT - based on @VGR's comment below, I have updated the example. Now it expects a timezone display name - for example "America/New_York" :

public String getTimezoneAbbrev(Date date, String tzName) {
    TimeZone tz = TimeZone.getTimeZone(tzName);
    boolean isDaylightSavings = tz.toZoneId().getRules().isDaylightSavings(date.toInstant());
    return tz.getDisplayName(isDaylightSavings, TimeZone.SHORT);
}

It works (I have test cases), but is there a less verbose way?

(I did not find any code examples which use Date as their starting point).

Answer 1

Something like the following?

import java.util.Date;
import java.util.TimeZone;

public class Main {
    public static void main(String[] args) {
        System.out.println(TimeZone.getDefault().getDisplayName(TimeZone.getDefault().inDaylightTime(new Date()),TimeZone.SHORT));
    }
}

Answer 2

I have accepted Arvind's answer because he pointed out TimeZone.inDaylightTime() - but for the record, here is the updated version of the method in my question.

It is more concise now:

// tzName is a timezone display name, such as "America/New_York"

public static String getTimezoneAbbrev(Date date, String tzName) {
    TimeZone tz = TimeZone.getTimeZone(tzName);
    return tz.getDisplayName(tz.inDaylightTime(date), TimeZone.SHORT);        
}

Warning

I also found answers in this question helpful - because if you type a non-existent timezone display name (or ID) when using TimeZone.getTimeZone(), Java will return "the specified TimeZone, or the GMT zone if the given ID cannot be understood" (emphasis mine). See here .

Typing...

America/New-York

...instead of...

America/New_York

...is enough to cause problems.

Therefore...

We should usejava.time.ZoneId instead. This will throw an error for an invalid/unknown ID.