It is important to ensure that the date and time are correct. Security Certificates are issued for a set period of time (sometimes only 12 months) and, if your date is say just a few days behind the certificate's expiry/renewal date, you are then trying to enter the site on a date that the Certificate has expired. ....I hope my comments make sense.!!
However, as indicated by Tuvok, if date and time are correct, there is something else wrong. Which in my opinion is something haywire with the site. In which case note what O_G said above..."Continue there at your own risk".
Incidentally there is another thread running along on this matter.:-
http://www.theanswerbank.co.uk/AB-Editors-Blog/Question1323695.html
Hans.