My T keeps track of the time. I always wear a watch but I never look at it during my session and I appreciate knowing that I don't have to worry about the time. Most of the time my sessions last about 10 mn more than they should, a few times when I was talking about something very difficult they lasted twice the time they should have. I felt really bad for the person waiting after my appointment, but my T said I didn't have to worry about it, that he was the one responsible for it. It's good to just go in there and forget about the outside world. Most of the time I can sense when a session is going to end, by a change in his tone, or when he just sit back up instead of lying in his chair. End of the session is also the time when he talks the most, he lets me do the talking (or at least try as I'm very silent) and then he talks, he explains things to me or develops things I've said and tells me what I need to work on sometimes.
I don't think it would be fair for your T to substract those 15 minutes as it wasn't your decision to go over time and in my opinion keeping track of the time is the T's job, not ours.