The lives of Siddhar Nagkmur (a regretful alien time traveler) and Stacey Papandreon (a tired immortal) converge for the second time since 522 AD; throw in some more aliens and a desperate need to repair the timeline to complete the story.
Nagkmur finds a chronology on the Internet and searches out a year halfway between the present and their encounter in sixth century Constantinople. The quickest way to identify when things went awry, he tells her, is to work by halves. If AD 1300 is undisturbed, the change came later; otherwise, earlier.

Variants

(1)
  1. “Nexus” by Michael F. Flynn, in Analog, March/April 2017.
  2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . written by Michael F. Flynn