Java 11 , get timezone of users local machine

We are developing applications where dates are stored in GMT , and need is to display date and time according to users local clock timezone

Java 11 , when i change timezone , its requires JVM restart so its taking old timezone of during start of server.

Is there any ways to determine timezone of machine ( OS) without restarting JVM?

Code

Calendar cal = Calendar.getInstance();
        System.out.println(" -- " + TimeZone.getDefault());
        long milliDiff = cal.get(Calendar.ZONE_OFFSET);
        // Got local offset, now loop through available timezone id(s).
        String[] ids = TimeZone.getAvailableIDs();
        String name = null;
        for (String id : ids) {
            System.out.println("ID  : " + id);
            TimeZone tz = TimeZone.getTimeZone(id);
            System.out.println("TZ : " + tz.getRawOffset());
            if (tz.getRawOffset() == milliDiff) {
                // Found a match.
                name = id;
                break;
            }
        }
        System.out.println("Name " + name);

Above code works OK as standalone, but when intregrated in web application ( JVM ) its KO.

java.time

You are using terrible date-time classes that were years ago supplanted by the modern java.time classes defined in JSR 310.

Avoid setting default time zone

No need to set the current default time zone of your JVM. Doing so is risky as you immediately affect all other code in all threads running in all apps in that JVM. And doing so is unreliable in that any code in any thread of any app in that JVM can change the zone at any moment behind your back.

Instant

Capture the current moment as seen in UTC (an offset from UTC of zero hours-minutes-seconds) using Instant class.

Instant now = Instant.now();
System.out.println( "now = " + now );

>now.toString() = 2023-02-16T06:24:54.944850Z

ZoneId

Time zones are represented by the ZoneId class.

Use real time zone names in the format of Continent/Region, such as Africa/Casablanca, Europe/Berlin, Pacific/Auckland. Never use 2-4 character pseudo-zones such as CST, IST, etc.

Set < String > zoneNames = ZoneId.getAvailableZoneIds();
for ( String zoneName : zoneNames )
{
    ZoneId z = ZoneId.of( zoneName );
    ZonedDateTime zdt = now.atZone( z );
    System.out.println( "Now in zone " + z + " is " + zdt.toString() );
}

Now in zone Asia/Aden is 2023-02-16T09:24:54.944850+03:00[Asia/Aden]
Now in zone America/Cuiaba is 2023-02-16T02:24:54.944850-04:00[America/Cuiaba]
…
Now in zone Pacific/Guadalcanal is 2023-02-16T17:24:54.944850+11:00[Pacific/Guadalcanal]
Now in zone Europe/Athens is 2023-02-16T08:24:54.944850+02:00[Europe/Athens]
Now in zone US/Pacific is 2023-02-15T22:24:54.944850-08:00[US/Pacific]
Now in zone Europe/Monaco is 2023-02-16T07:24:54.944850+01:00[Europe/Monaco]

Current moment as seen by user

You asked:

>need is to display date and time according to users local clock timezone

Capture the current default time zone of the JVM at runtime.

ZoneId z = ZoneId.systemDefault() ;

However, if critical, your app should confirm the user's intended time zone.

Adjust our Instant object to that zone. Same moment, different wall-clock time.

ZonedDateTime zdt = instant.atZone( z ) ;

Or skip over the Instant object to get ZonedDateTime directly.

ZonedDateTime zdt = ZonedDateTime.now( z ) ;