See the original puzzle
The clock is a digital clock.
Click for image
One of the LCD lights is broken. So, for example, when it is 8:18, it reads 9:18, an hour fast. Similarly, at 6:00, it reads 5:00, an hour slow. In the hours of 2, 10, and 12, the display will not make any sense at all. But most of the day (14 out of 24 hours), the clock reads correctly.
As a side note, I've just added new graphics to the margins of my blog. I hope no one minds that I basically spoiled the solution to one of my puzzles, but that was over a year ago, and I like the graphic.