secondTimeline objects created as part of delayTimeline are never stopped
which leads to memory growth and potential OOM.

Modifying delayTimeline the following way fixes the issue:

   144 @Override public void handle(ActionEvent event) {

if (secondTimeline != null)
   secondTimeline.stop();

   145 secondTimeline = new Timeline();
   146 secondTimeline.setCycleCount(Timeline.INDEFINITE);
   147 secondTimeline.getKeyFrames().add(
   148 new KeyFrame(Duration.seconds(1), new EventHandler<ActionEvent>() {
   149 @Override public void handle(ActionEvent event) {
   150 refreshClocks();
   151 }
   152 }));
   153 secondTimeline.play();
   154 }

I am not sure this is correct fix from DigitalClock functionality, so please
provide another one in case this is the case.