Basic Booking System Help!

Hi,

I work for a small sound company and we are wanting to create a basic rental system which will allow us to book equipment for specific jobs. Once I scan a piece of equipment, the bar code is decoded and the cell shows what the kit is, when you enter the out and in dates the relating date in a calendar page will go red and read unavailable, however if I try to book the same bit of kit again, the calendar does not change and shows the equipment as available.