Getting Physical – Box2D - Cocos2d-x by Example Beginner's Guide - page 311

Time for action - creating a timer
We create timers in pretty much the same way we create our main loop.
1. First, we add a second scheduled event by adding this line to our GameLayer
constructor:
this->schedule(CC_SCHEDULE_SELECTOR(GameLayer::ticktock),
1.5f);
2. 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).
3. 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));
}
}