Doesn't even need to be sci-fi hijinks....

Find a small bookmaker, who relies on his electric wall clock. Splice into his power line a "waveform recoder".

Several hours before a major sporting event, swing his power from 60Hz to 40Hz. Running at 40Hz for 6 hours gives only 4 hours elapsed on the clock. Now you are two hours ahead of the bookmaker.

Find out the results of the match. Drop by the bookmaker, and make your bet - because you know the outcome. Now switch him to run at 80Hz for 6 hours, and his clock will be back in sync.

Go in the next day and collect your winnings. Do NOT repeat.