Calculate the days remaining until an SSL/TLS certificate expires.
SSL/TLS certificates have a fixed validity period. When they expire, browsers show security warnings and connections may fail. Planning renewal before expiry (with a buffer for processing time and potential issues) prevents outages. Most CAs now limit certificates to 90-398 days.
Days remaining
days = expiry_date - today
Renewal deadline
renew_by = expiry_date - buffer_days
Browsers display a security warning (often blocking the page), automated API clients reject the connection, and search engines may penalize the site. Some services like Let's Encrypt auto-renew, but any failure in that process can still cause an outage.
30 days is a common minimum — enough time to notice renewal failures, resolve domain validation issues, and propagate the new certificate across load balancers and CDN nodes. Critical production systems often use 60+ days.