How to stop the timer in Angular?

How to stop the timer?

All code with implementation is available on StackBlitz.

If you click on button 'Go', the timer will start. I want to pause or stop this time, but how?

Main function is:

startTimerForSingleTraining(timer2) {
  console.log('timer 2', timer2)
  // if(timer2 === 'finished') {
  //   this.countDownForSingleStartTraining.unsubscribe();
  // } 
  this.countDownForSingleStartTraining = timer(0, this.tick)
    .subscribe(() => ++timer2.timer); 
}