You haven't told us what your structure is. I assume that you have a customer table, an item table (which may be books, CDs, or whatever the library lends), and an instance table. The instance table represents a physical object since the library may have multiple copies of each item and you need to know which copy is on loan. So, in the instance table, you would keep, customerID and ExpectedReturnDt. When the item is loaned out, these values are populated and when the items are returned, the fields are reset to null. If you want to keep history, you would create a separate history table at this time to log who borrowed something, what they borrowed, and how long they kept it.
With that type of process, you would identify the status of an item by checking its CustomerID field. If it is null, the item is in the library, if the CustomerID contains a value, the item is out on loan.
