If it’s due back at 2, it means 2 o’clock. As a question, you would ask, (as the borrower) “When is this book due back?” As the giver, you would ask, “ Can you return it by the due date?” You can’t say, “Make the due back at 2.” You’d say, “Please return it by (at) 2.”

"Due back at 2" - I am not sure if you mean 2 o'clock, or the 2nd of the month? 'at 2' means 'at two o'clock'. If the book is due on the second of the month, then it is due 'on the second'. Note the difference - 'at 2' and 'on the 2nd'. the library generally sets the return date. If you want to ask for a longer period you could ask ''can I have an extension on the due date'? Does that answer your question?

"I borrowed a book from a library and the agent told me that the due back *time* is at 2 p.m" OR "I borrowed a book from a library and the agent told me that the *book* is due back at 2 p.m" OR "I borrowed a book from a library and the agent told me that the *deadline to return the book* is 2 p.m" "Please *bring the book* back *by* 2" OR "Please make the deadline" "due" = must be done by a deadline "back" = returned "due back" = must be returned by a deadline -- You can say "Can you extend the due back *time*?"

"The due back is at 2" sounds really odd to me. Are you sure they didn't say "It is due back at 2"? 'due back' isn't a noun phrase, it's two words.