Appointment - add "invisible" times to services

I want to accomplish this:
Offer customers five times a day to amke an appointment:
10:00, 11:30, 15:00, 16:30, 18:00

The appointments are 60 minutes each and I need 30 minutes before or after for preparation and clearing of the room.

I have two difficulties:

1. If I set duration of services to 60 minutes the schedule shows options every hour. So I would need an option to add a "hidden" 15 minutes addition before and after every service time.

2. I cannot set specific times because on the settings page it is only possible to have a fixed time frame over the whole day (set to 90 minutes it offers these times:10:00, 11.30, 13:00, 14:30, 16:00, 17:30)