Check any site's SSL certificate.

Questions

What does this SSL checker actually test?

It connects to the host over HTTPS and reads the certificate the server presents, then reports the issuer, the valid-from and valid-to dates, how many days remain, and the subject alternative names the certificate covers.

Why does my certificate show as invalid in a browser but fine here?

Usually the certificate itself is fine but the chain is incomplete — the server is not sending the intermediate certificate. Browsers on some platforms cannot fill that gap. Reinstall the full chain bundle your certificate authority provided.

How soon should I renew before expiry?

Renew at least 14 days out, and automate it if you can. Let's Encrypt certificates last 90 days and are designed to auto-renew at 60 days; a paid certificate usually lasts a year, so set a calendar reminder as a backstop.