It's a pretty major plot point that the Dark Dimension is timeless. Strange alters this with his time loop spell after he arrives.
But when you observe the Dark Dimension before he casts that spell, time is still quite evidently in effect. The planets(?) move, for instance, Strange moves through the dimension, and Dormammu himself does not seem very put off by this at all, seeming to progress through time forwards as standard.
I'm further confused by Strange's statements during the time loop. He tells Dormammu that everybody on Earth will live, even though when he returns, they are still in the same moment and are clearly being affected by the loop. It seems to me that they won't live at all, since they are frozen in time.
Furthermore, why does Dormammu bother with Kaecilius at all? Shouldn't he know, since he is timeless, that he won't succeed?