Mr. Zhang bought a new watch and checked it with the wall clock at home. He found that the watch went three minutes slower than the wall clock at home. Later, he checked the wall clock at home with the standard time of the radio station and found that the wall clock walked three minutes faster than the standard time of the radio station. 
Mr. Zhang inferred that his watch was on time.
Mr. Zhang's inference is wrong, because the wall clock is three minutes faster than the standard time, which is the standard three minutes, and the watch is three minutes slower than the wall clock, which is three minutes non-standard