Safari warns certificate is invalid

Every time I try to access a site, Safari tells me that the certificate is revoked or expired. But the certificate is not expired. When I instruct Safari to trust the certificate, it appears to do so, but then it still won't let me access the site. I chatted with the site's IT and it suggested wiping my cache. But that will wipe my passwords to. I'm at a loss, and I really need to access this site. It's Princeton University's library.