I was able to go into app + php and change the service time

I was able to go into app + php and change the service time duration to every 5 minutes, but it now when i selet at 35 min appointment it starts the time on the front end. E.g. it starts the appointment at 9:35 instend of starting at 9:30. How do i adjust the php so that it adds the 5 min to the end of the time slot?