Java Calendar.getInstance() changing the default timezone in Linux

Question

Please see my code as below :

SimpleDateFormat sd = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);  
sd.setTimeZone(TimeZone.getTimeZone("GMT"));
System.out.println(sd.format(calendar.getTime()));

My Windows system's default time zone is EDT at this moment and time when I ran this code is (28/09/2016 12:27 PM)and when I run this code in the system the output I get as below -- which is intended (EDT to GMT Conversion) :

28/09/2016 04:00:00   

But when I run this on server (Red Hat Enterprise Linux Server release 5.11) the output I get as below :

28/09/2016 00:00:00

When I ran the below command to the Linux shell

date +%Z

It returned below output

EDT

So, I am not able to understand why the conversion did not happen. Also, I have a piece of code like below :

SimpleDateFormat sd = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
Calendar calendar = Calendar.getInstance();
sd.setTimeZone(TimeZone.getTimeZone("GMT"));
System.out.println(sd.format(calendar.getTime()));

Which returned below output (ran it on 28/09/2016 12:36 PM) in the same Linux Server with the intended output that is converted it to GMT

28/09/2016 16:36:46   

This code is part of a J2EE application running on WebLogic 12c. Please share if you have any clue, what might have caused the above mentioned scenario. Thanks.

Answer 1

You're relying on the default time zone. Specify both time zones, and the conversion should work.

28/09/2016 12:27:00 -> 28/09/2016 16:27:00 +0000

And here's some test code.

package com.ggl.testing;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

public class TimeZoneConversion {

    public static void main(String[] args) {
        SimpleDateFormat sdInput = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss");
        sdInput.setTimeZone(TimeZone.getTimeZone("US/Eastern"));
        SimpleDateFormat sdOutput = new SimpleDateFormat(
                "dd/MM/yyyy HH:mm:ss Z");
        sdOutput.setTimeZone(TimeZone.getTimeZone("GMT"));

        try {
            String dateString = "28/09/2016 12:27:00";
            Date inputDate = sdInput.parse(dateString);
            System.out
                    .println(dateString + " -> " + sdOutput.format(inputDate));
        } catch (ParseException e) {
            e.printStackTrace();
        }

    }

}