The cert_expiry module reads every file an ssl_certificate names, when the
configuration is read and again on a timer, and reports how long the
certificate still has: at warn level once it is close to its end, at error
level once it has passed it.  cert_expiry_warn sets the threshold,
cert_expiry_check the interval, and $ssl_expiry_days carries the days left
into a log format.

This port only installs the module sources.  The module itself is built by
the nginx ports when their CERT_EXPIRY option is enabled.