It is well known that a stopped clock gives the exact time twice a day, while a clock that gains or loses time may not be right more than once over a period of months.
My clever father adjusted his clock to give the correct time at least twice a day, while running at the normal rate.
Assuming he was not able to set it perfectly (a reasonable assumption) how did he do it?
If the clock ran backwards at the same rate, then it is evident that the clock will show the correct time at least twice a day.
Consequently, the father altered the mechanisms of the clock in such a manner that it ran backwards.