Frame zero is a mythical creature.

I've decided that the entire last post was a result of semantic confusion. Keyframes and frames are different things; a frame is a measure of elapsed time, and a keyframe is a sample of animation at an instant in time, so it makes perfect sense to say an animation with four sampled keyframes actually has three frames; because frames are just seconds in disguise, just another way of expressing the elapsed time. The trick is to keep t accurate and all you have left is interpolation.