Near the end of Johnny Hart's life, he revealed B.C. to take place in a post-apocalyptic world by having new character Anno Domini have copies of, among other things, phone books and The Bible in his cave. So it was Earth All Along sorta.
The infamous Halloween 1989 Garfield storyline (Oct 23-28) has the title character finding himself in a seemingly post-apocalyptic world (or at least living in a long-abandoned house). This has led fans to wonder if this is the true setting of the strip.