I have my service times set for 5 minute so I can have

I have my service times set for 5 minute so I can have space between apointments like book at 30 minute treatment but have a 5 minute padding so my service time is acutally 35 minutes. One of your staff gave me the code to make that happen. is there a way to keep the 35 min service time but show up on my calendar as 15minute booking slots. right now people can book every 5 minutes. i need it to show every 15 minutes.