Recently Mac users started seeing "Your connection is not private" error for even the websites they trust. There is a workaround to get rid of this message (just click on the 'Advanced' button on the bottom and then click on the link to the website) but it is annoying to see the warning ERR_CERT_DATE_INVALID for almost all the websites you access.
Root cause: If you are using an outdated Operating System, you will notice this issue. This is due to the expiration of the root certificate on September 30, 2021 issued by Let's Encrypt who is one of the largest non-profit certificate authority.
You will see a warning that the root certificate is not trusted but ignore this for now. Next step is to double-click the certificate and mark it as trusted so that OS X will trust the root CA’s signed certificate. You can see the root PEM certificate in the key manager now trusted by OS X.
If the root certificate is expired, the clients including your browser will start showing the warning to users.
How to fix this issue?
Ideally you should be upgrading your operating system to the latest version so that you will get all the security updates to keep your system safe. For some reason if you are not able to upgrade your operating system, continue reading.
Download Let's Encrypt Root Certificate:
Go to Let's Encrypt website and download the ISRG Root X1 certificate (isrgrootx1.pem). Now double-click the pem file and install the certificate as system (refer the screenshot below).
Also you can add certificates to the keychain using the below command:
sudo security add-trusted-cert -d -r trustRoot -k /Library/Keychains/System.keychain isrgrootx1.pem
