Working hours- off by 10 minutes

I put 50 minutes as my time base, as all my dance classes are 50 minutes. However, when I try to put my working hours in, everything is off by 10 minutes, because it won't let me choose the actual time I start. (3:10pm) it will only allow me to choose 3:00pm, because it starts the 50 minute intervals at midnight. So, if people book their appointments on this plugin then they will all think their appointment starts 10 minutes earlier than it does. Is there a way to change this in the working hours? I was going to change it to 10 minute base time, but then people would be able to book at all kinds of times, and I need them booking the specific times I have set aside for classes to start, i.e. 310, 400, 450, 540, 630, 720, 810, 900, 950. Or appointments would overlap, which cannot happen. Ideas?
Thank you,
Erin