The reason I cant base it on time is because I am animating the the speed at which the object moves. I had it set up to animate based on time before, the problem with that was that i could not make the animation speed up without making the contents within the animation speed up along the way.
I am animating clips moving horizontally on the screen. I want them to move faster at one point and then slow down, without speeding up the playback of the clips.