I remember reading recently that humans started dating documents relatively late in history, and that even as late as in the Renaissance most people wouldn't h

I remember reading recently that humans started dating documents relatively late in history, and that even as late as in the Renaissance most people wouldn't have been aware of the current calendar year. [Ong 1982] Given that paleographists estimate how old ancient papyri are by means of dated documents I wonder: Where these documents explicitly dated or are datings mostly extracted from the context (e.g., names of rulers / geographical references)? Are "documentary" papyri always reliable for comparisons? In any case very interesting.