Slots currently use a notion of guard time to prevent an appointment that was just cancelled from being snapped up by another student. There is no such feature for signing up for an unassigned slot.

If there was a "must schedule before" guard time, it could serve this purpose as well as give the teacher certainty that no one would sign up two minutes before a slot. So if a teacher had set a "must schedule before" time of 12 hours and saw there were no appointments that day, they could choose not to come in to the office.