Hi. 
I have appointments for 30 and 15 minutes.
The 30 ones start on every whole and half hour. How can i make them start every 15min? 

I tried setting duration to 30 and slot step to 15. Doing this enables overlapping of appointments.

Example of what I want.
30m appointment at 9.00
15m appointment at 9.30
30m appointment at 9.45