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Java 一段时间代码 28800000 的问题？

//显示时间
Date d=new Date(System.currentTimeMillis()-startTime.getTime()-28800000);
这句话啥意思？为什麽-28800000？

下面是全部代码∶
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.*;
import java.util.Date;
import java.text.SimpleDateFormat;

public class StopWatch extends JFrame {
JButton btnStart, btnStop;
JLabel label;
Timer timer;

public StopWatch() {
label = new JLabel("0000:00.000");
btnStart = new JButton("start");
btnStop = new JButton("stop");
final int delay = 100;
final Date startTime = new Date();
final SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss.S");
final Action taskPerformer = new AbstractAction() {
public void actionPerformed(ActionEvent evt) {
// 显示时间
Date d = new Date(System.currentTimeMillis()
- startTime.getTime() - 28800000);
System.out.println(d);
label.setText(sdf.format(d));
label.repaint();
}
};
btnStart.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
startTime.setTime(new Date().getTime());
timer = new Timer(delay, taskPerformer);
timer.start();
}
});
btnStop.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
if (timer != null && timer.isRunning())
timer.stop();
}
});

Container c = getContentPane();
c.add(label, BorderLayout.NORTH);
c.add(btnStart, BorderLayout.CENTER);
c.add(btnStop, BorderLayout.SOUTH);
}

public static void main(String[] args) {

javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});

}

private static void createAndShowGUI() {
StopWatch window = new StopWatch();
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.pack();
window.setVisible(true);
}
}

关键字 : public, timer, import, label, window, startTime, final, btnStop, btnStart, StopWatch, ActionEvent, getTime, actionPerformed, System, JButton, BorderLayout, ActionListener, 28800000, 显示时间, taskPerformer

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2009-06-04T11:59:13+0000