My time is at 15 minutes past too. It used to be on the hour but he asked to move it because he needed more time between the two appointments. Personally I'm glad he doesn't rush himself and takes the time he needs.
I manage time. My T has a clock in my view and I moan at him when it's out of sync with the other clock in the room (it's a wind up clock). I stop when the hour is up. If there was no clock in my view I would probably either ask for one or take a watch.
As for going over, that is on her. T1 used to do that often, but personally I like fixed times better because I know where I stand and how to pace the session.
Also, don't worry about the other client, she probably leaves large time gaps between sessions, which I personally take as an indicator of good self-care.