Countdown timer in JavaScript

This thread has been very helpful for a game I'm creating in storyline. Thank you. 

I am using the timer as a countdown for a question. I have managed to set the timer to start as the timeline starts on the slide. I have also added a slide layer to show once the timer reaches zero. However, how do I manage to pause the timer once the user has selected an answer? 

I am not proficient in Javascript, so please explain in simple terms. Thank you kindly.