Here's how the booking calendar works:
You set blocks of availability.
You can have appointments within these blocks (but not exceeding them.)
You have different available appointment durations (for each of your appointment types,) for example: "50 minutes"
You have rules that govern bookings. These are set in your calendar/booking settings.
If you have a "start time restriction" (set within your settings) on the quarter hour, that means allowed start times would be at x:00 x:15 x:30 x:45
If you have a minimum break between appointments of 11 minutes (as an example, you can set this to anything you want)..
Assuming availability on Monday (and no other bookings) from 9am-12pm, when someone selects a "50 minute" appointment., that's going to show available start times as follows:
9:00, 9:15, 9:30, 9:45, 10:00, 10:15, 10:30, 10:45, 11:00
You wouldn't have an available start time at 11:15 because you can't do a 50 minute session starting at 11:15 because that would put the end time at 12:05 -- and your availability only lasts until 12:00.
Now, let's say you have a booking at 10:15 for a 50 minute session.
Now available start times that would be displayed would be only 9:00am
If you start a session at 9:15am, that means that it ends at 10:05am. You have a minimum break between appointments of 11 minutes. That means that the earliest the next session could start would be 10:16. However, you have a booking already at 10:15. So a 9:15 start time would cause a 1 minute overlap (because of your 11 minute break requirement.) If your break were set to 10 minutes, then the 9:15 start time would be visible because that would end your break at 10:15, which is a legal start time for your next appointment.
Your 10:15 appointment ends at 11:05, then we have the 11 minute break, so that means the next possible start time is 11:16. However, you can only start appointments on the quarter hour, so the next available slot would be 11:30. However, since your availability is only until 12:00pm, then you can finish the appointment in time.
The effect of minimum break between appointments at the end of your availability
Also, take note, if you have a break between appointments of 11 minutes and you had an appointment starting at 11:00, that slot would be allowed because the actual appointment would end at 11:50. So even though the "break" would end at 12:01 (outside of your availability,) break time doesn't matter. In other words, break time is only relevant between appointments -- not at the end of an appointment with nothing following.
Why you can't see available dates or times
You won't see available dates and times until you select a duration. This is because there is no way to know if a date or time is available until you know how long the desired session will be.
The dates and times are the result of session duration and the rules you have set.
On the booking calendar, you have to select an appointment type, then select a duration, then you'll be able to see available dates, then available times. It's a rules based system so if you don't select an appointment type, then you don't know what durations you have, without durations, the system can't calculate time slots that would accommodate.
Contact us at firstname.lastname@example.org if you have more questions!