public class MyProg extends JFrame {
   
  private JLabel TimeLabel;

 public MyProg() {
      
	   
    // заголовок окна 	   
     super("Окно"); 
    // делаем окно максимальным 
     setExtendedState(JFrame.MAXIMIZED_BOTH);
     
     
//////////////////////создали листенер ////////////////////////////////// 
ActionListener timerlistener = new ActionListener() {
public void actionPerformed(ActionEvent e)  {

 long curTime = System.currentTimeMillis();
 String curStringDate = new SimpleDateFormat("dd.MM.yyyy HH.mm.ss").format(curTime);
 buttonsPanel.remove(TimeLabel);
 TimeLabel=new JLabel("          "+curStringDate);
 buttonsPanel.add(TimeLabel);
 validate(); 
                                                          }
                                                                  } 
/////////////////////////////////////////end listener////////////////////////////////


   Timer timer1 =new Timer(1800000,timerlistener);
   timer1.setInitialDelay(3000);
   timer1.start();                                                       
   long curTime = System.currentTimeMillis(); 
   String curStringDate = new SimpleDateFormat("dd.MM.yyyy HH.mm.ss").format(curTime);
        TimeLabel=new JLabel("          "+curStringDate);
        
        /* Кнопки обновить, закрыть  */
        buttonsPanel = new JPanel(new FlowLayout());
        buttonsPanel.add(TimeLabel);
        add(buttonsPanel, BorderLayout.SOUTH);
	      
}