Time marches relentlessly onward like grains of sand falling through an hourglass, once each has passed the threshold, it will never pass that way again. But time precognition, chronomancy, and time travel are common themes in fantasy fiction, as are the notions of fate, destiny, and even the timestream itself. Yet such things are rarely dealt with in most games.
The reason is obvious time is a tricky, extremely powerful, wibbly-wobbly concept. Even in a game where players with modern day sensibilities are asked to put themselves in the roles of medieval characters and process the mechanics of magic, psionics, and infinite planes of reality, it seems beyond the pale to ask them to un-know certain sets of experiences simply because time travel has caused them to cease to exist, and to keep track of what is and isn't true when jumping back and forth between timestreams and potential realities.
Yet, there's a reason that chronomancy is one of the topics that fans most often request supplements for. The idea of playing with time is fun!
The Genius Guide to the Time Thief presents a new base class, the time thief a character whose abilities all focus on various aspects of swimming the wrong way up the timestream, if only for a little ways. She plays with probability, can peer into the future to avoid bad decisions, and steal time from her own (and others') futures to fuel her reality-bending powers. Most of all, she's just a lot of fun to play.