Java时间格式转换，如何获得时区？

//我实现一个时钟窗口程序给你了，好让你更容易理解，希望对你有帮助。
import java.awt.*;
import java.awt.event.*;
import java.util.*;

//世界时钟
public class TimerTest {
public static void main(String[] args) {
new TimerTestFrame("世界时钟");
}
}

class TimerTestFrame extends Frame {
/**
*
*/
private static final long serialVersionUID = 1L;

public TimerTestFrame(String s) {
super(s);
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
dispose();
System.exit(0);
}
});
setLayout(new GridLayout(2, 6));

ClockCanvas clk1 = new ClockCanvas("北京", "GMT+8"); // 创建时钟
ClockCanvas clk2 = new ClockCanvas("巴黎", "GMT+2");
ClockCanvas clk3 = new ClockCanvas("华盛顿", "GMT-4");
ClockCanvas clk4 = new ClockCanvas("洛衫矶", "GMT-7");<