Although your question looks more like a generic javascript question than WordPress specific and would probably be better suited for another forum like stackoverflow.com, I would say the problem in your code is that you’re using id="countdown" for all the countdown elements. You should use class="countdown" if you want to use the element multiple times on a single page. You can also check that you’re getting all the right elements by doing a console.log(elems); after the document.querySelectorAll.
