Changing timezone without changing time in Java

calendar, datetime, java, timezone

Solution

I've decided to reparse the datetime string received with the correct time zone set. This should also consider daylight saving:

public class DateTest {

    private static SimpleDateFormat soapdatetime = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS");

    /**
     * @param args
     */
    public static void main(String[] args) {
        TimeZone oztz = TimeZone.getTimeZone("Australia/Sydney");
        TimeZone gmtz = TimeZone.getTimeZone("GMT");
        Calendar datetime = Calendar.getInstance( gmtz );

        soapdatetime.setTimeZone( gmtz );
        String soap_datetime = soapdatetime.format( datetime.getTime() );
        System.out.println( soap_datetime );

        soapdatetime.setTimeZone( oztz );
        datetime.setTimeZone( oztz );
        try {
            datetime.setTime(
                    soapdatetime.parse( soap_datetime )
            );
        } catch (ParseException e) {
            e.printStackTrace();
        }

        soapdatetime.setTimeZone( gmtz );
        soap_datetime = soapdatetime.format( datetime.getTime() );
        System.out.println( soap_datetime );
    }
}

Problem

I'm receiving a datetime from a SOAP webservice without timzone information. Hence, the Axis deserializer assumes UTC. However, the datetime really is in Sydney time. I've solved the problem by substracting the timezone offset: ``` Calendar trade_date = trade.getTradeDateTime(); TimeZone est_tz = TimeZone.getTimeZone("Australia/Sydney"); long millis = trade_date.getTimeInMillis() - est_tz.getRawOffset(); trade_date.setTimeZone( est_tz ); trade_date.setTimeInMillis( millis ); ``` However, I'm not sure if this solution also takes daylight saving into account. I think it should, because all operations are on UTC time. Any experiences with manipulating time in Java? Better ideas on how to solve this problem?

Original source