Time is endless. However the universe resets over the span of trillions of years. Each universe is mostly the same, since it all comes from the same source, but they have slight differences, mostly due to the source's life experience and random chance. The presence of gems however isn't slight. In a previous universe, future humans created the gems as helpers. White Diamond was the crowning achievement of humanity. She was powerful enough to survive the death and rebirth of the universe, and she alone rebuilt the gem civilization. Why do you think Steven Universe's world has so many differences from ours? Why do you think gem influence is infinite? Why is White so knowledgeable about humans, to the point of saying Steven was acting like a child, a completely foreign and difficult to understand concept for most gems? The series takes place billions if not trillions of years in our future, in a reborn universe, in a reborn galaxy, on a reborn Earth, with a reborn humanity. Only White is aware of this.