Why SSL certificates need checking
An SSL/TLS certificate expiring unnoticed is one of the most common causes of a site suddenly showing security warnings to every visitor - browsers refuse to load it normally, and the fix (renewing and reinstalling the certificate) only becomes urgent once it's already broken for real users. Checking the expiry date ahead of time, along with who actually issued the certificate, is a quick way to catch this before it becomes an outage.
What the certificate details actually tell you
Beyond the expiry date, the issuer field shows which Certificate Authority vouches for the certificate, the Common Name confirms which domain it's actually issued for, and the Subject Alternative Names field (when present) lists every additional domain or subdomain the same certificate also covers - useful for confirming a single certificate properly covers all the subdomains you expect it to.
Why checking days remaining matters, not just expired-or-not
Catching a certificate with, say, 10 days remaining gives you time to renew calmly before it becomes an emergency - and that habit matters more every year, since the CA/Browser Forum (the industry body that sets these rules for every publicly trusted Certificate Authority) is in the middle of a multi-year phase-down of maximum certificate validity: 200 days starting March 2026, 100 days from March 2027, and just 47 days by March 2029. Let's Encrypt, which issues a large share of the web's certificates, is moving even faster on its own timeline, planning to switch to 45-day certificates by February 2028. Shorter lifespans mean more frequent renewals, which is exactly why periodic rechecking is genuinely worthwhile rather than a one-time setup task.
A quick way to verify a certificate after renewal
After renewing and reinstalling a certificate, running this check again confirms the new expiry date and issuer actually took effect on the live site - a useful final step to catch a renewal that technically completed but didn't get properly deployed to the server.