main.dart

class TimerAppState extends State<TimerApp> {
  static const duration = const Duration(seconds:1);

  int secondsPassed = 0;
  bool isActive = false;

  Timer timer;

  void handleTick() {
    if (isActive) {
      setState(() {
        secondsPassed = secondsPassed + 1;
      });
    }
  }

  @override
  void initState() {
    timer = Timer.periodic(duration, (Timer t) {
      handleTick();
    });
    super.initState();
  }
  
  @override
  Widget build(BuildContext context) {
    
    int seconds = secondsPassed % 60;
    int minutes = secondsPassed ~/ 60;
    int hours = secondsPassed ~/ (60*60);

    return MaterialApp(...)
  }
}