Java : Displaying time in UTC

Question

I am trying following piece of code using the Java Calendar API

class TimeIssue 
{
    public void showUTCTime(long millis, final String msg)
    {
        Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
        cal.setTimeInMillis(0);
        System.out.println(msg + "(UTC):" + cal.getTime());
    }

    public void showLocalTime(long millis, final String msg)
    {
        Calendar cal = Calendar.getInstance();
        cal.setTimeInMillis(0);
        System.out.println(msg + "(LOCAL):" + cal.getTime());
    }

    public static void main(String[] args)
    {
        long epochSec = 0;
        TimeIssue obj = new TimeIssue();

        obj.showUTCTime(epochSec, "EPOCH");
        obj.showLocalTime(epochSec, "EPOCH");
     }
}

When I execute this program, I get the output as ( My Time Zone is GMT+5:30)

EPOCH(UTC):Thu Jan 01 05:30:00 IST 1970
EPOCH(LOCAL):Thu Jan 01 05:30:00 IST 1970

I have 2 concerns here

• Should not the first line of the output be Jan 01 00:00:00 UTC 1970
• Why I am getting IST both the times even when I have set the time zone

Could you please suggest what am I missing here ?

Abhishek

Answer 1

The Calendar.getTime() method returns the Date object, which is just the number of milliseconds since epoch. Printing that instance invokes the Date#toString() method, which uses the system default timezone to format the date. Giving the Calendar instance a timezone, doesn't associate the resultant Date object with that timezone. In fact, Date object has no concept of timezones.

What you want is to format the date yourself using DateFormat, and set the timezone in that. Change your UTC method to this:

public void showUTCTime(long millis, final String msg) {
    Calendar cal = Calendar.getInstance();
    cal.setTimeInMillis(0);
    DateFormat dateFormat = new SimpleDateFormat("EEE MMM dd HH:mm:ss zzz yyyy");
    dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
    System.out.println(msg + "(UTC):" + dateFormat.format(cal.getTime()));
}

Answer 2

The problem is that you're setting the time zone by calling obj.showUTCTime(epochSec, "EPOCH"); and it's saved after this. You need to set it again in showLocalTime to have the other time displayed in another time zone.

The first line of output is UTC 00:00:00 1970 in your time zone, that is 5:30

Answer 3

The accepted answer is correct.

Avoid java.util.Date

Working with date-time and time zones is much easier if you use a competent library. The java.util.Date and .Calendar classes are notoriously troublesome. Use either Joda-Time or the java.time package in Java 8 (inspired by Joda-Time).

Joda-Time

Example code in Joda-Time 2.5. We instantiate a pair of DateTime objects that represent the same moment in the timeline of the Universe but as seen from two different wall-clock-times (UTC and India).

DateTime dateTimeUtc = new DateTime( 0, DateTimeZone.UTC );
DateTimeZone timeZoneKolkata = DateTimeZone.forID( "Asia/Kolkata" );
DateTime dateTimeKolkata = dateTimeUtc.withZone( timeZoneKolkata );