Duke's entire life is a demonstration of this sentence. In fact the feat of sword-swallowing is also just that, you know you did okay yesterday, you might or might not be there tomorrow to swallow the swords, it all depends on whether you can manage to do so today without damaging yourself.
As for myself, I guess I am too worried about the future. I reminisce of the past, but try not to dwell too much on that. However, the future has a way of scaring me! Guess I should really learn from Duke, but then again, not at all bothering about the future does not seem like a wise choice, either.