Six Months, Three Days: A Tor.Com Original / Charlie Jane Anders

Doug and Judy have both had a secret power all their life. Judy can see every possible future, branching out from each moment like infinite trees. Doug can also see the future, but for him, it's a single, locked-in, inexorable sequence of foreordained events. They can't both be right, but over and over again, they are.

Obviously these are the last two people in the world who should date. So, naturally, they do

Six Months, Three Days is the winner of the 2012 Hugo Award for Best Novelette.