Animorphs takes place in the present time. They even mention events that are happening at the time the book is published, like Episode I: The Phantom Menace coming out on DVD. Animorphs isn't really a long story, but a bunch of little ones (somewhere around 41 when this was written). This makes it hard to predict their future. It doesn't seem likely that they will ever defeat the Yeerks until K. A. Applegate gets tired of writing Animorphs. But it is likely that some kind of drastic change will be made, like them joining forces with a band of Andalites, or something of that sort.

In Animorphs there is something called the Time Matrix, which can be used to travel through time. But K. A. Apllegate stays very realistic, taking into account that you can't just drive into the future, or drive into the past, and that everything is in motion. In book seven, they are shown that their fight against the Yeerks is hopeless, yet then they destroy the Kandrona, changing that future. In Megamorphs #3, they troop all over history, each event changing the next. In the end, they go back in time and eliminate the problem that forced them to go back in time in the first place. This makes it so that they really had never gone back in time, yet they have the memories of it. The future is a very complex thing.

One last thing: in Animorphs they support superstring theory, which means that if we could see down to the subatomic level, we would see tiny strings. Through the complexities of this, it it known that the universe has ten dimensions, yet the other six are curled up into each other so that we can't see them. Notice I said the other six, which means that there is a fourth dimension: time. Understand that superstring theory is just a theory, and there has been no actual proof of it.