Gravitational waves are 'ripples' in space-time caused by some of the most violent and energetic processes in the Universe, like tremendous supernova explosions or black hole collisions. These cataclysmic events disrupt space-time sending 'waves' of undulating space-time in all directions away from the source at the speed of light.
So, Beatrice Westover uses these waves as a means to travel through space and time. But wait a minute; aren't those waves hard to find? And when you do find one, doesn't it shoot by at the speed of light?
Yes, and yes.
Albert Einstein predicted the presence of gravitational waves, but scientists have only recently confirmed his prediction. We are inundated with grav waves, all the time, but they are really hard to detect, only the largest of them -- like those described above -- have been detected by the finest equipment designed to do so (see LIGO or Laser Interferometer Gravitational-Wave Observatory).
Theoretical Physicist Westover has found a way to amplify smaller, more plentiful waves, making more available for time travel. This "amplifier" works like a capacitor, requiring time to re-charge. It is this re-charging time that determines both the leave time and the return time for the chrononauts.
Craig retired in 2015 and has been writing ever since. And boy, is his hand tired.