I can't get votes to expire

Hello I'm having some trouble getting my tallied clicks to expire... Please see the website here: http://www.bigfreelinks.com

As you can see, I would like the highest visited websites in our directory to get pushed up to the top of the list on our homepage; and the lifetime of any website clicks should lasts for only 12 hours, but I've not been able to get the clicks to expire, they only continue to add up!

Any help with achieving this would be great! :) thankyou to anyone who may be able to help!