sorry for responding late, and although it seems that you have found a solution I'd like to post a possible solution for people who find this thread using the search function afterwards:

Basicly all you do is first setting the starting time and then subtract from it as long as it is above zero.
My example will use "seconds" as time measurement.
The variable "total_secs" stores the time in seconds which is still left.
My example does not take care of converting this to minutes.
Code:

---

// !!! Aware: C-Script Code !!!

var total_secs = 0;

function timer_start(_seconds)
{
  total_secs = _seconds;

  while(total_secs > 0)
  {
    wait(-1);
    total_secs -= 1;
  }
}

function timer_stop() //use this function to force the timer to stop
{
  total_secs = 0;
}

// Start the timer like this. (example uses 2 minutes as start value)
timer_start(120);

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