I'll give you a great tip right now:
If you're counting time in the game, don't do coroutines or invokes or anything, just put this in your Update function:
float timer; (not in update)
{
timer += Time.deltatime;
timerText.setText(timer);
}
By using Time.deltatime, every frame it adds the amount of time that frame took.
That's how you make a simple timer/counter!