Countdown Timer

Hi,

I am working on a system simulation, and would like to have some sort of continuous countdown timer (around 2 - 2.5 min).  Ideally, when the timer ends that would trigger a feedback slide; however, this is not a must.  Additionally, I would like to have this timer be visible throughout the simulation

I have read a few different threads that talk about this, and one that seemed promising was the one I found in the attached file.  I am, however, having trouble reverse-engineering it.

Any thoughts?

Appreciate the help.