I recently added a couple of countdown timers to the right side of my blog for some major events coming up. If you're wondering how I did it, I did what any self-respecting coder would do: I checked Google.
Unfortunately I didn't come up with anything that was very easy to use. So I took what I found and modified it. The comments in the code should make it self explanatory. The function can be called any number of times for any number of countdown timers on the page. Just set a different var name and target.