How to make a timer with stopper functionability?
A key for start, stop, and reset?
Timers are your friend, you can start, pause and reset them:
https://docs.unrealengine.com/latest/INT/Gameplay/HowTo/UseTimers/Blueprints/index.html
From timer header you can get current state of timer, which you can format (using TimeSpan type? From Seconds | Unreal Engine Documentation) an disiplay as a text via Tick or via binded variable
Since they count down, you need to make timer with huge time number and use this node to read the time: