OK, not made it among the first three (I'll blame the time-zones), but no big deal, participating is everyting.

So, I decided to trying to get the Most Ancient Fic points and chose a fandom classic: Irene Dutch's "Firestorm" from 1999.
You go here:
http://lcfanfic.com/stories/1999/html/firestor.html
or here:
https://archiveofourown.org/works/35356177
(the latter link contains the FDK comment)
EDIT:
OK, and here's a quote to prove th estory qualifies:
QUOTE
"All right, young lady. You told the receptionist that you needed to speak to me about Superman. Please go ahead."

Lee took a deep breath. "Dr. Klein, you're going to think I'm crazy, but … I'm from the future. "

He started to laugh. "How'd you get here - by bicycle?"

"No, by time machine of course," Lee retorted. "Please Dr. Klein. Hear me out before you have security throw me out."

"All right. I'll listen, if you promise to leave quietly when you're done."

"Dr. Klein, that's a promise I won't have to keep. You're not going to want me to leave in a couple of minutes."

"We'll see. Please go ahead with your … explanation."


--
That should do it.