If I understand you correctly,
The glint /glare is controlled by a Timeline in this example.
It is just an animation of a group's ( with rectangle inside) origin and motion.
The animation's easing is linear by the way.
Select the Timeline 'glint' and then you will be able to run the play head back and forth to see where it starts and finishes . You will be able to edit the animations on the timeline like any other..