LOL. Do you think Oswald returned every book that he checked out precisely two weeks later? A logical inference strongly suggests that this reflects the due date.
The information provided by the WC in the OP is incomplete, and the wording is ambiguous.
"Return date" suggests the date the books were returned. "Due date" would be a different matter.
Reading the entire Commission Exhibit on this matter, we learn that the New Orleans public library system (supposedly) does not keep detailed records on patrons' activities.
No further effort seems to have been made to discover the actual library activity of Oswald. Why?