Certificates expire because a shorter validity window limits the damage a compromised, mis-issued, or outdated certificate can do. If a private key leaks, or a certificate was issued based on information that's since changed (a domain changed ownership, an organization no longer exists), expiry is a backstop that eventually invalidates it even if nobody catches the problem manually.
The trend is toward shorter, not longer
Maximum certificate lifespans have been repeatedly shortened industry-wide — from years, down to 398 days as the current common ceiling for publicly trusted certificates, with further reductions actively discussed. Let's Encrypt certificates are just 90 days specifically to force automation, on the theory that automated renewal is more reliable than a human remembering a yearly task.
The practical takeaway
Don't fight the trend by trying to get the longest-lived certificate possible — set up automated renewal instead (ACME clients, cPanel AutoSSL, or your platform's built-in renewal) so expiry is a non-event rather than an annual scramble.
What would happen if certificates never expired
Without expiry, a certificate issued for a domain that later changed ownership, or a certificate whose private key quietly leaked years ago, would remain valid and trusted indefinitely with no natural mechanism to force revalidation — expiry acts as a backstop that limits exactly this kind of long-tail risk even when nothing else catches it.
How to stop thinking about expiry entirely
The practical fix isn't fighting the trend toward shorter validity periods — it's automating renewal entirely with an ACME client like Certbot, or your host's built-in free SSL, so that expiry becomes a non-event handled automatically rather than a recurring manual task you need to remember.
The historical trend in maximum allowed validity, briefly
Maximum publicly trusted certificate validity has been repeatedly shortened by industry policy — from as long as five years in the early 2010s down to the current 398-day ceiling — with further reductions toward even shorter windows an actively discussed possibility within ongoing CA/Browser Forum policy conversations.
What a maximum validity period actually constrains
The maximum validity period set by browser policy caps how long any single certificate can be valid for from issuance, currently 398 days for publicly trusted certificates — a CA cannot issue a certificate exceeding this window regardless of what a customer might request or be willing to pay for, since browsers would simply refuse to trust a certificate issued outside the policy.
Why Let's Encrypt chose 90 days specifically, shorter than the maximum allowed
Let's Encrypt deliberately set validity well below the maximum permitted window specifically to force automation as the only practical renewal approach, reasoning that automated renewal is more reliable at scale than expecting manual renewal from a very large, diverse population of site owners.
The single best thing to do about this entire topic
Rather than tracking expiry dates manually, set up automated renewal once through an ACME client or your host's built-in free SSL, and this entire category of risk becomes something you never need to think about again.
What the practical difference is between a 90-day and a 398-day certificate for a typical site
For a site with automated renewal already in place, the specific validity period makes essentially no practical difference — both renew automatically well before expiry. The difference only matters operationally for sites still relying on manual renewal, where a shorter window means more frequent manual intervention required.
A closing thought
A quick closing thought: once automated renewal is properly configured, expiry becomes something you never need to think about again — the entire practical concern this article addresses simply disappears behind reliable automation.