Hello.
At the Booking Calendar Business Small and Business Medium versions are possible to make the bookings for the specific time slot.
Please note, if some visitor will reserve the specific time slot at selected date at specific booking resource (calendar), this timeslot become unavailable for the other visitors at this selected date in this booking resource.
In the free version of Booking Calendar is possible to make the bookings only for the specific day(s).
You can test Booking Calendar live demos at this page.
Also you can check features, that available in the different versions at this page.