How to show 30 min time increments in schedule?

I'm using Appointments + on a client's site. She offers a variety of services which range in time from 30 to 150 mins, and her time base is set to 30 mins in the settings. I'd like the list of available times to show in 30 min increments on days that she has working hours. For example, if her working hours are between 10AM and 6PM, she wants available times to show as 10:00, 10:30, 11:00, 11:30, etc. regardless of the length of time a particular service takes.

Currently if a service is 60 mins for example, it will only show times at 10:00, 11:00, 12:00, etc. and a client can't book an appt beginning on the half hour, ie. 10:30 or 11:30.

How can I change that?