How do I resolve this error message: NET::ERR_CERT_DATE_INVALID?

Keith is on the right track. Pretty much every report here of seeing that advisory is from users running the badly obsolete macOS 10.11 El Capitan or older. There is a reason that affesta many sites..

A major supplier of browser security certificates has allowed some of its certs to expire. For Mac users this affects OS versions with outdated browsers. Here is that supplier's statement on the subject:

https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

Older devices and/or their OS versions may not support modern technologies that the cert's replacement uses. If you tell us your Mac's sub-model from "About this Mac" we can give your very specific recommendations regarding your possible upgrade paths. To get a newer version of Safari usually requires upgrading to a newer macOS version.

Unfortunately, as certs are usually purchased by sites and a big seller of them has abandoned support for outdated browsers, a site owner may have trouble updating any offending cert.

Any 21.5-inch iMac can run OS 10.13 High Sierra and, for now, that should update Safari enough to get around the problem. Any apps you have that are running under 10.11 should run fine under 10.13.

Is this from a website?

You might have to get the owner of the site to update their certificate, or use a browser like Chrome which will let you bypass the security standards.