In the story, the time traveler travels to the sixties, I believe, and posts an ad somewhere along the lines of:
"Remember Reaganomics? VCRs? Perestroika? Call me at KL-123."
Obviously, only another time traveler who'd experienced the eighties would recognize those terms ('VCRs' may be misremembered, but you get the point). I'm pretty sure about the "Reaganomics" term, which will date its writing to most likely in the eighties, certainly no earlier than 1981.