I still have no real notion of what you are trying to do. What do you mean by 'countdown'?? Do you mean that someone visits a url and in their browser there's this ticker counting down? If so, that's javascript...you would need to retrieve the value from your database and convert it to some HTML/Javascript that creates a meaningful date object and then write a script (or steal one) to display a countdown timer.
If, on the other hand, you want things in your DB to expire, there are two ways to do it...one is to check dates on user queries...like SELECT FROM DB WHERE my_datetime > '2007-01-01 12:00:00'. If you need something to happen* whether or not users are visiting, then you need to create something like a crontab (a unix thing...not sure what the equivalent is in windows).
