var counter;
var counter_state = 0; //0 = not counting, 1 = counting

function counter_set(_state,_value)
{
  counter = _value;
  while(counter_state)
  {
    counter -= 1;
    counter_state *= sign(counter);
    wait(-1);
  }
}

function counter_add(_value)
{
  counter += _value;
}

//Start the counter like this (e.g. 20 seconds):
counter_set(1,20);

//End the counter like this:
counter_set(0,0);

//Add something to the counter (e.g. +40 seconds):
counter_add(40);

//player did something wrong, give him less time (-20):
counter_add(-20);