I have an old iMac that has been sitting unused upstairs for awhile, that I decided to finally get rid of. Before putting it on Craigslist, like any good computer owner, I wiped the drives and went to reinstall the most recent version of OS X/macOS that this old machine would support. In this case, this was El Capitan.
But when I went to install, I kept getting an error about the OS not being able to install. Popping the log window open, I found an entry called CSSMERR_TP_CERT_EXPIRED. This would seem to be a prime suspect.