Slasher
is it just one site that gives you problems? If the problem isn't due to your computers date being sent incorrectly, its normally due to
1) the owners of a site forgetting to renew their site security certificates when they run out
2) a certification authority withdrawing a certificate because they've found that either the site or the owner is dodgy
3) a cert being withdrawn because it was hijacked by a third party
If the date on your computer is correct, let us know which sites are giving the problem. You can PM me if the sites are too embarrasing for anyone else to know about......