Clock.java

package dalydesigns.swing;

import java.awt.event.ActionEvent;
import java.awt.event.ItemEvent;

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

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Toolkit;

import java.awt.event.ActionListener;
import java.awt.event.ItemListener;

import javax.swing.Timer;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JComboBox;

public class Clock extends JFrame {
    private Toolkit toolkit;
    private TimeZone tz;
    private SimpleDateFormat format;
    private JLabel time;
    private JComboBox timezones;

    public Clock() {
        setTitle( "Digital Clock" );

        tz = TimeZone.getDefault();

        format = new SimpleDateFormat( "h:m:s a" );
        format.setTimeZone( tz );

        JPanel main = new JPanel();
        main.setLayout( new BorderLayout( 10, 10 ) );

        time = new JLabel( format.format( new Date() ) );
        time.setFont( new Font( "Courier New", Font.BOLD, 14 ) );
        time.setHorizontalAlignment( JLabel.CENTER );
        time.setForeground( new Color( 50, 50, 25 ) );

        timezones = new JComboBox( TimeZone.getAvailableIDs() );
        timezones.setSelectedIndex( -1 );
        for( int i = 0; i < timezones.getItemCount(); i++ ) {
            if( timezones.getItemAt( i ).toString().equals( tz.getID() ) ) {
                timezones.setSelectedIndex( i );

                break;
            }
        }
        timezones.addItemListener( new ItemListener() {
            public void itemStateChanged( ItemEvent e ) {
                String zone = (String)timezones.getSelectedItem();
                tz = TimeZone.getTimeZone( zone );
                format.setTimeZone( tz );
            }
        } );

        main.add( time, BorderLayout.CENTER );
        main.add( timezones, BorderLayout.SOUTH );
        main.setBorder( BorderFactory.createEmptyBorder( 10, 10, 10, 10 ) );
        add( main );
        pack();

        toolkit = getToolkit();
        Dimension screenSize = toolkit.getScreenSize();
        setLocation( ( screenSize.width - getWidth() ) / 2, ( screenSize.height - getHeight() ) / 2 );
        setDefaultCloseOperation( EXIT_ON_CLOSE );

        Timer t = new Timer( 1000, new ActionListener() {
            public void actionPerformed( ActionEvent e ) {
                time.setText( format.format( new Date() ) );
            }
        } );
        t.start();
    }

    public static void main( String[] args ) {
        Clock clock = new Clock();
        clock.setVisible( true );
    }
}