Appointments + Time Increments and Service Duration

We are using appointments + for a client booking calendar.

We require the timeslots to be booked in half hour increments and the service duration to be for an hour

i.e. time slots are:
9:00 - 9:30
9:30 - 10:00
10:00 - 10:30

i.e. service duration would be:
9:30 - 10:30 or 11:00 - 12:00

We have set the 'Time base (minutes)' to 30 minutes, and the Service duration to 60 minutes which should solve what we are asking for. The problem is when changing the service duration to 60 minutes the time increments on the calendar change to be an hour apart and not 30 minutes...

Is there a way to solve this?