Countdown Timer

[Newcore]

Hi Everyone,
New to the community and UIFlow / Coding in general.
Trying to create a countdown timer that starts when button A is pushed and stop when timer hits "0"
So far I can make the timer start by pushing button A and stop by pushing button B.
How do you make a timer function stop when "0" is reached and reset?
Here is a screen shot of what I have so far.