I just reread this and noticed this piece.
In my experience, if the gal is having a good time or she's pretending to be and she cut's it short, it's because she's not totally into you or she literally has a legit excuse.
On the other hand, if time is standing still for her and you cut it short then it may be communicating subliminally that you are getting bored or not really into her that much.
A girl is going to keep wanting to be with you and not keep track of time if she's really into you. I've had first dates that's lasted anywhere from 4-8hours. Generally if a girl doesn't even bat an eye at the time after a couple hours, there's a good chance you can venue change, kino/makeout, invite her back to your place.
OP, do you find yourself initiating the ending of a date? Maybe this is why you get these questions often.