I guess I found the easiest countdown timer on the web, namely Andrew Urquhart's DHTML Easy Countdown — v1.53. All you have to do is to copy the javascript provided in the downloaded file onto your hosting server, call this javascript at the beginning of your page, then use the 'countdown1' selector id in any div, span or other tag, and finally provide the date and time within and you're good to go! ;)
Example code to call the javascript (put this between the 'head' tags):