It looks like there is already an api for just that:
 new CountDownTimer(30000, 1000) {
     public void onTick(long millisUntilFinished) {
         mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);
     }
     public void onFinish() {
         mTextField.setText("done!");
     }
  }.start();
Reference:
http://developer.android.com/reference/android/os/CountDownTimer.html 
solved How to use Chronometer on Android?