You can create fixed times in the calendar that can be booked, regardless of the duration of the services.
For example: you can only book at 9 am, 11 am, 1 pm and 3 pm.
So no other times should be proposed outside the fixed times where someone can book (e.g. 9:15 a.m.) should not be an option, even if there is room for another appointment at that time.
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