Less than full 1/2 hour of time assigne
System changes computer assignment slots at 15 minutes after the current slot time. For example, at 3:13, time slots for 3:00 are assigned; whereas at 3:16, time slots for 3:30 are assigned.
Example:  Patron signs up at 3:13 pm and is assigned a computer from 3:00 - 3:30. However, the patron wants to have a full half hour on computer. Patron can either use computer now for 17 minutes and then sign up again, or else wait two minutes (until 3:15) at which time system will make assignments for the 3:30 time slots. Before signing up second time (for a 3:30 time slot) patron must cancel (item "4" on menu) current signup (otherwise she will get error message ("can't be signed up more than once for same time slot")).