Presuming that you are using a form for data entry the simplest way to do this would be as follows.
Create a text box on your form called for example 'Days Remaining', make it's data type a number.
Set the control source of this text box to be whatever your date due back field is called minus your date loaned field. Like this:
=[Date Due Back]-[Date Loaned].
The box will then countdown the days remaining for each loan. You can then set up some sort ofr message box to trigger when a loan reaches 0 days.