Date and TimeZone in Java

The date handling in Java is much more trickier than what it looks on surface, setting TimeZone in Calendar object does not make any difference. Date and Calendar objects are always in machine’s time zone, irrespective of what one tries to tell these classes.

In web application at times we need to convert the dates in user preferred time zone while displaying on the browser. Usually all dates are stored in one timezone to make it easy to change them in user's preferred or some other timezone. The general practice is to store dates in GMT. Take an example of a web application where a user creates an event say Delhi Daredevils vs. Rajasthan Royals and sets its start and end date / time (April 10, 2009 07:00 PM to 11:30 PM). The browser will send date / time as String e.g. servelet_url?start=04/10/200919:00&end=04/10/200923:30.

The venue of the event is in IST while the time zone of the server where web application is running is PST. When a date is constructed from the request parameters then it will be always in JVM's timezone. A date can be changed to another timezones for storing in DB or displaying on browser using following utility methods-

    // Change a Date to GMT
    public static Date toGMT(Date date) {
        return changeTimeZone(date, "GMT");
    }

    // Change a date to GMT from a given timezone
    public static Date toGmtFromZone(Date date, String fromZone) {
        TimeZone pst = TimeZone.getTimeZone(fromZone);
        return new Date(date.getTime() - pst.getRawOffset());
    }

    // Change a date in another timezone
    public static Date changeTimeZone(Date date, TimeZone zone) {
        Calendar first = Calendar.getInstance(zone);
        first.setTimeInMillis(date.getTime());

        Calendar output = Calendar.getInstance();
        output.set(Calendar.YEAR, first.get(Calendar.YEAR));
        output.set(Calendar.MONTH, first.get(Calendar.MONTH));
        output.set(Calendar.DAY_OF_MONTH, first.get(Calendar.DAY_OF_MONTH));
        output.set(Calendar.HOUR_OF_DAY, first.get(Calendar.HOUR_OF_DAY));
        output.set(Calendar.MINUTE, first.get(Calendar.MINUTE));
        output.set(Calendar.SECOND, first.get(Calendar.SECOND));
        output.set(Calendar.MILLISECOND, first.get(Calendar.MILLISECOND));

        return output.getTime();
    }

Internally Date class holds the TimeZone reference but that is always machine time zone that’s why while invoking Date.toString() it prints local time zone, which can be overlooked safely. Take this example where we have a date in IST, which later converted in GMT but Date.toString() always prints 'IST' with it because my machine's timezone is IST.

IST: Sat Mar 07 22:38:16 IST 2009
GMT: Sat Mar 07 17:08:16 IST 2009