Later Than You Think by Fritz Leiber - It's much later. The question is ... how late?

Obviously the Archeologist's study belonged to an era vastly distant from today. Familiar similarities here and there only sharpened the feeling of alienage. The sunlight that filtered through the windows in the ceiling had a wan and greenish cast and was augmented by radiation from some luminous material impregnating the walls and floor. Even the wide desk and the commodious hassocks glowed with a restful light. Across the former were scattered metal-backed wax tablets, styluses, and a pair of large and oddly formed spectacles. The crammed bookcases were not particularly unusual, but the books were bound in metal and the script on their spines would have been utterly unfamiliar to the most erudite of modern linguists. One of the books, lying open on a hassock, showed leaves of a thin, flexible, rustless metal covered with luminous characters. Between the bookcases were phosphorescent oil paintings, mainly of sea bottoms, in somber greens and browns. Their style, neither wholly realistic nor abstract, would have baffled the historian of art.

A blackboard with large colored crayons hinted equally at the schoolroom and the studio.

In the center of the room, midway to the ceiling, hung a fish with irridescent scales of breathtaking beauty. So invisible was its means of support that-also taking into account the strange paintings and the greenish light-one would have sworn that the object was to create an underwater scene.

The Explorer made his entrance in a theatrical swirl of movement. He embraced the Archeologist with a warmth calculated to startle that crusty old fellow. Then he settled himself on a hassock, looked up and asked a question in a speech and idiom so different from any we know that it must be called another means of communication rather than another language. The import was, "Well, what about it?"