If you'd like to set up specific start times for your appointments, there are two options:
First, head to scheduling limits to change the number you see in the "Display start times for appointments..." settings to the timing you would like to set for each appointment start time.
Your second option would be to go to Availability and list exact start times as a list , rather than a time range, for your availability. In this example you would enter in a list of exact start times: 9am, 9:30am, 11am, 12:30PM. That will look like this: