If a service starts at 13:00 and there are free spaces, a user can not make a booking once the time reaches 13:00. An admin can make a booking retrospectively, but Eventbrite and other scheduling services allow users to continue making bookings after the start time.
We are using bookings to reserve a space for students (roughly 800 spaces per hours) and when people arrive after the start time, without a booking we went to let them in but for contact tracing purposes we need a record of who was where. It is too onerous for the admins to book all late starters manually!
You can try and set the minimum lead time for a service to -1 hour etc but when you press save, it reverts to zero.