It is clear that a broken clock, or even a clock that is stopped, will not be able to properly tell time. With this, the clock would be considered unreliable, as pointed out in the phrase. However, a broken clock whose minute and hour hands are stopped will still tell time right twice a day, as they will be stuck on the correct time, once in the morning and once at night. Over time, people began to use comparison to show that even something unreliable will sometimes still be right.

Source: Business Writing