Animated background for Responsive Design project

Could you avoid expressions altogether and use track mattes instead?  Or is it that the animation is all part of the same layer and the out sequence is already set in the same layer, so you need to looping to end always on the last 25th frame?

If it is the latter, I played with one of Dan's expressions (thank you as always Dan) and came up with this.  I made an essential graphic of a photon torpedo (0 to 5 seconds) which looped until a second position keyframe, and then played an explosion.

var endLoop = 5;
var endExplosion = 10;

var p = transform.position;
var val = 0;
if(p.numKeys > 0){
    n = p.nearestKey(time).index;
    if(p.key(n).time > time) {
	    n--;
    }
    if(n == 2){
        t = p.key(n).time;
        if(time < t+endExplosion){
            val = linear(time, t, t+endExplosion, endLoop, endExplosion);
        } else {
            val = endExplosion;
		}
    } else {
		loopTime = (time - key(1).time)%endLoop;
		val = valueAtTime(key(1).time + loopTime);		
	}
}
val

Last thought, have you tried loopIn() instead of loopOut(), that always ends the loop on a keyframe and can then continue playing.

The problem is that loopOut() will cause the expression engine to ignore everything previous to that in the expression. You need to create your own loopOut functionality, probably by using the the JavaScript modulo operator (%). I'm not sure this is exactly what you're looking for, but try replacing your last two lines with this:

TimeChange % framesToTime(289)