Thanks for posting the pics.
Hey, no thanks to me earlier?
I think the 3 biggest reasons that the clocks are not a continuity error is that:
1) The Preview splices Locke then the "other" clock, then Locke, and so on. In the real episode with the hatch clock we see it count down from directly behind Locke so the camera angle isn't even close.
2) As the pictures show it isn't close to the same clock and they showed the "other" clock countdown from 9 to 0 and the hatch clock countdown as well, so it wasn't just a chance using of the wrong picture, i.e the editor grabbed an old clock they decided to change.
3) The Preview was create after the episode was shot, so they would have to go out of their way to use an "other" clock instead of the one shown right in the episode.
Again, this is not 100% as people do make mistakes, but if it was it would be an egregious (thanks to Jason Wood for reminding me about that word) error to say the least.