Let's take a look at Microsoft IE pre-installed CAs. You can check this in the browsing directory through Tools|Internet Options|Content|Certificates|Trusted Root Certification Authorities:
Root certificates do have an expiry date, but they tend to have exceptionally long validity times (often between 10-20 years). As part of a browser or operating system update patch, you will get fresh root certificates before the old ones expire. Lacking a CAs root certificate, no browser would know whether to accept an SSL certificate issued by that CA. These SSL certificates are known as single root certificates.
