The certificate is expired, or not yet valid.
WHAT IT MEANS
The current date falls outside the certificate's validity period.
THE USUAL CAUSE
An expired certificate. Almost always a manually installed certificate that nobody renewed, or an AutoSSL renewal that has been failing for weeks while the notification emails went unread.
FIXING IT
- cPanel > SSL/TLS Status. Check the expiry date shown.
- Tick the domain and run AutoSSL.
- Refresh after a few minutes.
If AutoSSL fails, click the domain for the reason and work through the AutoSSL troubleshooting articles.
FOR A PURCHASED CERTIFICATE
Renew it with the issuer, then install the new certificate and its intermediate bundle through cPanel > SSL/TLS > Manage SSL sites.
IF IT AFFECTS ONLY ONE VISITOR
Their computer clock is wrong. A device with the date set years ahead or behind rejects every certificate on the internet.
PREVENTING IT
Use AutoSSL, which renews itself. If you must use a purchased certificate, calendar the expiry with a reminder a month ahead.
