Theoretical physicist Sandra Kobayashi took the afternoon flight from Kyoto, where she gave a talk to a sleepy, yet courteous audience of professional colleagues about the theory of general relativity and its limitations in addressing dark matter. Owing to the idiosyncrasies of time zones, she left Kyoto at 5 P.M. on Monday afternoon and would arrive home in San Francisco at 11 A.M. on the same day.

Shortly after takeoff, she mulled the phenomenon of arriving earlier than she left, which was really just a quirk in accounting, when a stab of insight jolted her from her seat. Trembling violently, she scrawled the calculations into her yellow notepad. She landed, six hours earlier, in the future.

Her theory was reviewed, re-reviewed, and re-re-reviewed. Labs across the world conducted tests, and each test confirmed what she discovered over the Pacific Ocean, sometime between today and yesterday: a process to travel backward in time.

The phenomenon worked much like time dilation, but in reverse. Rather than aging more slowly as one travels forward in time, Sandra developed a process to move backward in time, but at the cost of aging faster.

The dilation factor was three-hundred-to-one, which meant that for every day a person traveled backward in time, they aged roughly a year.

One year in total isolation aboard a cramped shuttle moving near the speed of light. Just to take back one day.

Thirty years of your life to repeat a month.

Procrastinators, test-takers, and criminals; the very clumsy and the very apologetic; the wealthy indecisive and the desperate poor; everybody who lost somebody they loved:

Suddenly, they had a very difficult calculation to perform.