What this tool does
Enter a domain or an HTTPS URL. The tool opens a TLS connection to it and reads the certificate that the server presents. It shows who issued it, when it is valid from and until, how many days are left, and the list of names the certificate covers (Subject Alternative Names).
Why check
- A certificate that has expired shows visitors a full-page browser warning.
- A certificate that doesn’t cover the name you are using (for example
www) also fails. - Auto-renewal can quietly stop working, so a quick check before the expiry date saves a bad morning. Let’s Encrypt certificates last 90 days, which makes this more important.
Reading the result
The days left figure is calculated from the certificate’s “valid to” date. The issuer is the certificate authority. The SAN list is the real list of covered names. Modern browsers use it and ignore the older Common Name field.
What it does not check
It reads the leaf certificate only. The connection is made without verifying trust, so a self-signed, expired or otherwise untrusted certificate is still displayed, and the tool does not warn you that browsers would reject it. It doesn’t validate the full chain, check revocation, test supported TLS versions or ciphers, or grade your configuration. A site can show a valid certificate here and still have a missing intermediate certificate that breaks some devices. For a deeper audit use a dedicated TLS testing service.
If a site is behind a CDN, you see the CDN’s certificate, not the one on your origin server.
Results are cached for 30 minutes, with up to 12 checks a minute. Redirect and HTTPS behaviour are covered by Server Status Checker.
Does it check the whole certificate chain?
No. It reads the certificate the server presents, with its dates and names, and does not verify that browsers would trust it. Chain and revocation checks need a more thorough tool.
Why do I see a different certificate than on my server?
If the site is behind a CDN or proxy, the CDN presents its own certificate.
Can it test TLS versions and ciphers?
No. It only reads the certificate.
How soon before expiry should I renew?
Many people renew with at least two to four weeks left. Auto-renewing setups usually renew at 30 days.
What certificate details does it show?
The issuer, the valid-from and valid-until dates, how many days are left, and the list of names the certificate covers.