The Slow Path, or Two and a Half Centuries in Two and a Half Days, by Amy Wolf.
PG-13. Ten, Reinette, Mickey, Rose.
AU from Girl in the Fireplace.
There’s a definite frown on Reinette’s face now. “You’re going to go meet her.” Her voice is chill. “You’re going to leave.”
He takes a long look at her face, and swallows. “Well, not for a long time.”
“How long?”
“Two and a half centuries?” He does a quick count on his fingers. “Well, nearly.”
Great characterizations, lovely old school touches, and all the history! It's enough to make me squee, it is.