How do I stop the timer without the user noticing?

I’m having a problem with the countdown tool on this page

https://codepen.io/SitePoint/pen/NWxKgxN

initializeClock('clockdiv', deadline);

I can add a new timer as I want, but the old countdown continues. How can I stop or delete the current countdown?

initializeClock(‘clockdiv’, deadline); somehow I added new timer but 2 timers are running at the same time.