to display local date/time for different countries in java

Question

I have to display local date time for different countries in java.Currently I am setting zoneId for "America/New_York" and so only getting EST time for all the places on the server.How to achieve local date/time dynamically.

DateTimeFormatter globalFormat = DateTimeFormatter.ofPattern("dd-MMM-yyyy hh:mm a z");
DateTimeFormatter estFormat = DateTimeFormatter.ofPattern("dd-MMM-yyyy hh:mm a 'EST'");
ZoneId istZoneId = ZoneId.of("Asia/Calcutta");
ZoneId estZoneId = ZoneId.of("America/New_York");

Instant instant = Instant.now() ;
ZonedDateTime zdt = ZonedDateTime.now( estZoneId) ;
ZonedDateTime currentISTime = instant.atZone(istZoneId);                //India time
ZonedDateTime currentESTime = zdt.withZoneSameInstant(estZoneId);       //EST Time         

System.out.println("est time.............."+estFormat.format(currentESTime)); 

Answer 1

Make use of ZoneId.systemDefault():

public static void main(String[] args) {
    // take the instant
    Instant instant = Instant.now();
    // use it in system default time zone
    ZonedDateTime zdt = instant.atZone(ZoneId.systemDefault());
    // then in Asia/Calcutta
    ZonedDateTime currentISTime = instant.atZone(ZoneId.of("Asia/Calcutta"));
    // and in America/New York
    ZonedDateTime currentESTime = zdt.withZoneSameInstant(ZoneId.of("America/New_York"));
    // then print them all using the ISO format for zoned date times
    System.out.println("System default:\t" + zdt.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
    System.out.println("EST:\t\t" + currentISTime.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
    System.out.println("IST:\t\t" + currentESTime.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
}

On my system, this prints

System default: 2019-11-26T12:19:18.865+01:00[Europe/Berlin]
EST:            2019-11-26T16:49:18.696+05:30[Asia/Calcutta]
IST:            2019-11-26T06:19:18.865-05:00[America/New_York]

Find out what it does on yours and the remaining related ones. That's quite dynamical by relying on the system default (which may be manipulated under circumstances).

Answer 2

ZoneId with ZonedDateTime is what you want.

ZoneId zoneId = ZoneId.systemDefault();
Locale locale = Locale.getDefault();

You could offer a switch, which is useful for development too:

Set<String> availableZoneIds = ZoneId.getAvailableZoneIds();
Locale[] availabelLocales = Locale.getAvailableLocales();

Adding your own Locale might sometimes be required, especially for small countries or Esperanto. It requires some effort.

Usage of ZonedDateTime for the default:

ZonedDateTime zdt = ZonedDateTime.now();

A localized date/time/date-time formatting: DateTimeFormatter:

DateTimeFormatter.ofLocalizedDateTime(FormatStyle.LONG).withLocale(Locale.FRANCE);

(withLocale should you want to use a specific locale.)

Needless to say store all as universal coordinated time, Z, or ZoneId.of("UTC").