My google calendar suddenly won't sync with apple calendar due to certificate error

Click on "Show Certificate" -- it will probably say something like "this certificate is marked as trusted for..." and show some other URL or domain name, and you probably also see something about McAfee in the text. This means you're either using Centurylink or some other ISP which uses McAfee security. For fun, try to go to the URL or domain it lists, and your browser will probably be stopped saying the site "may be dangerous." That will confirm it. You can then close that browser window.

There is a problem there-- if you do have Centurylink, I'd report it to them. They won't take the word of just a few of us, but it is reading the wrong certificate for the Google calendar API, and my troubleshooting shows it's being led astray by something in McAfee.