Our primary method of measuring time is by the rotation of the Earth. Modern clocks that tell us 24 hour time derive from the consistent observation that it takes about that long for the sun to rise in the east, set in the west, and appear in the east again. So if you spin the earth in the opposite direction, it will make the sun set in the east, rise in the west, and set in the east again, and that means the clocks and all of time will move backwards.
The idea defies physics, as well as astronomy, but it is a popular method for Time Travel nonetheless. The results of actually trying to reverse the Earth's spin is likely to tear it apart, but whatever handwave sets that issue aside to discuss the effects of time travel instead.
