Hi Ang. I've found a few ways to deal with that, depending on the interaction.
You can make sure the end point is the same as the start point (if that makes sense in the interaction) and then jump to the time that the animation starts. You may want to turn easing off, depending on the effect you're going for.
Also, you can link animations (start one when another ends), and have the second one have the reversed motion path of the first. Then if you have a trigger to the jump to the start of the first motion path when the second ends, it will smoothly move back and forth.
Also, this may not be completely relevant but opens up some interesting possibilities: You can loop timelines of different lengths on multiple layer simultaneously, as long as they are not set to hide other layers.