A certificate expiry alert warns you in advance that a TLS/SSL certificate will soon stop being valid. It is triggered by comparing the certificate's notAfter date with the current date and a threshold you choose, for example 30, 14 and 7 days.
Why it matters
When a certificate expires, browsers show a full-page warning and API clients refuse to connect, which for most visitors looks like an outage. Even with automated renewal, things fail: a DNS change breaks validation, a cron job stops, or a new server never got the renewal client. Multi-stage alerts give you time to fix the cause calmly. Shorter certificate lifetimes make automation and checking more important, not less.
Example
A manual check that can be scripted:
echo | openssl s_client -connect example.com:443 -servername example.com 2>/dev/null \
| openssl x509 -noout -enddate
notAfter=Jan 14 09:00:00 2027 GMTAlert at 30 days to investigate, 14 days to escalate, and 3 days as urgent. Check every hostname and every endpoint, including mail servers and API domains.
Common mistakes
- Monitoring only the main site and missing subdomains, load balancers or mail ports.
- Checking the certificate on a CDN edge while the origin certificate quietly expires.
- Sending alerts to a former employee's mailbox.
Related
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