First, we add a second scheduled event by adding this line to our GameLayer constructor:

this->schedule(CC_SCHEDULE_SELECTOR(GameLayer::ticktock), 1.5f);

With this, we create a separate timer that will run the ticktock method every 1.5 seconds (I decided in the end that 1.5 seconds looked better).

The method keeps updating the value of the _time property and displaying it in the _timer label.

void GameLayer::ticktock(float dt) {
    if (_gameState == kGamePlay) {
        _time++;
        _timer->setString(std::to_string(_time));
    }
}