The Internet browser message "the security certificate has expired or is not yet valid" means one of two different things. Trustwave security software explains that the message can mean a security certificate has expired and should be updated as soon as possible. The other possible cause is that the clock on a computer may be set to the wrong time.
An expired security certificate means a computer's security needs to be updated. Security certificates are set up by servers that relay information to computers over the Internet. Renewing SSL certificates can be done online or over the phone.
If a computer's clock is off, synchronizing the time with a reliable source can solve the dilemma. The clock should be set to the right time zone, year and time of day. Exiting the Web browser and then attempting to go back to the website determines if changing the clock got rid of the message.
An AOL help page notes that an SSL certificate ensures information exchanged between an Internet browser and a server is private and integral. This type of software only works if the server and browser are in sync with each other. Error messages appear when a certificate is not accepted by a browser or when the certificate expires.