code: private void saveRegisterWatermarkTimer() {
  		long currentWatermark = timerService.currentWatermark();
  		// protect against overflow
  		if (currentWatermark + 1 > currentWatermark) {
  			timerService.registerEventTimeTimer(VoidNamespace.INSTANCE, currentWatermark + 1);
  		}
  	}
docs: Registers a timer for {@code current watermark + 1}, this means that we get triggered
  whenever the watermark advances, which is what we want for working off the queue of
  buffered elements.