Java - how to convert time zones to a different format?

Question

In my Java code I have appointments that have time zones related to each one.

The timezones are in the following format:

Europe/Zurich,
Indian/Antananarivo

I want to convert these to different formats. How can I convert these time zones into for example:

GMT
EST

Answer 1

You can use a java.time.ZoneId to parse them and display some corresponding short name:

public static void main(String[] args) throws Exception {
    // example String zones
    String zuerich = "Europe/Zurich";
    String antananarivo = "Indian/Antananarivo";
    // create ZoneIds from the Strings
    ZoneId zueri = ZoneId.of(zuerich);
    ZoneId antan = ZoneId.of(antananarivo);
    // print their short names / abbreviations
    System.out.println(zueri.getDisplayName(TextStyle.SHORT, Locale.ENGLISH));
    System.out.println(antan.getDisplayName(TextStyle.SHORT, Locale.ENGLISH));
}

Output:

CET
EAT

NOTE that this CET is probably not quite correct due to it being CEST at the moment.

EDIT

You can have the GMT representations if you take some instant into account:

public static void main(String[] args) throws Exception {
    // example String zones
    String zuerich = "Europe/Zurich";
    String antananarivo = "Indian/Antananarivo";
    // create ZoneIds from the Strings
    ZoneId zueri = ZoneId.of(zuerich);
    ZoneId antan = ZoneId.of(antananarivo);
    // create a formatter that outputs the GMT+/-XX:XX representations
    DateTimeFormatter gmtFormatter = DateTimeFormatter.ofPattern("OOOO");
    // or take "now" as a temporal reference and print the GMT representation per zone
    ZonedDateTime nowInZurich = ZonedDateTime.now(zueri);
    ZonedDateTime nowInAntananarivo = ZonedDateTime.now(antan);
    System.out.println(nowInZurich.format(gmtFormatter));
    System.out.println(nowInAntananarivo.format(gmtFormatter));
    // find out about the difference when the time switches from daylight saving
    ZonedDateTime sixMonthsLaterInZurich = nowInZurich.plusMonths(6);
    ZonedDateTime sixMonthsLaterInAntananarivo = nowInAntananarivo.plusMonths(6);
    System.out.println(sixMonthsLaterInZurich.format(gmtFormatter));
    System.out.println(sixMonthsLaterInAntananarivo.format(gmtFormatter));
}

prints

GMT+02:00
GMT+03:00
GMT+01:00
GMT+03:00

Looks like Zürich is switching an hour six months after now (July 16. 2021) but Antananarivo doesn't.