Thanks for coming to ITKE
Objects on the AS400 have a last used date. Unfortunately the object for the library does not get that field updated. Objects within the library have the date updated. A library is a special object. What I believe you are after would be the *LIBL (library List) the your processes are using. Library List can be set on a User Signed, a Job Description, an individual program and more.
Can you give us more information on why you need this data and how you applications are running.