How to dispose CountDownTimer

How to dispose CountDownTimer

I have main class called "MainActivity" and I'm lanuching it few times in
my App.
private static CountDownTimer timer;
private static final long startTime = 15 * 1000;
private static final long interval = 1 * 1000;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
timer = new StageCountDownTimer(startTime, interval);
}
private class StageCountDownTimer extends CountDownTimer {
public StageCountDownTimer(long startTime, long interval) {
super(startTime, interval);
}
@Override
public void onFinish() {
//STARTTING NEW ACTIVITY
}
@Override
public void onTick(long millisUntilFinished) {
}
}
Sometimes user need to close this activity before count down ends, and
return to the this activity again. And then new count down is launching
but the old one execute the code in onFinish() when previous count down
ends. Everything works great when I start this code once. How to
cancel/dispose/destroy this timer after exiting from activity? I tried
timer.cancel() and nothing happen.