Free tools
Check the active SSL/TLS certificate on port 443 of a domain: who issued it, which name it covers, and how many days remain before it expires.
It connects over TLS to the given domain and extracts the issuer, certificate name, and validity dates, including how many days remain until expiration.
A soon-to-expire certificate is one of the most common causes of outages on sites that used to work fine.
Frequently asked questions
People still say SSL out of habit, but the protocol actually in use today is TLS.
The tool will fail to connect properly or report an invalid certificate; it should be renewed as soon as possible.