option to set close date for hours

As we run short courses that people book late for, I would like to be able set this field so that enrollments can be accepted on the course on the day before the course but only up until say 3pm. This would give us sufficient time to complete the necessary preparation for the following day and no further bookings can be accepted after hours on the night before the course.    Is it possible to make this field accept numbers less than 1 or add an option to select hours instead of days?