I don't believe that is the correct place to check for this type of thing.

The functionality you're looking for is already available using openssl_x509_parse() to determine if the certificate is valid at a given timestamp as shown here:

<?php
$cert = ...; // get the cert from somewhere
$info = openssl_x509_parse($cert);
$time = time(); // or any other timestamp;
$isValid = ($info['validFrom_time_t'] < $time && $time < $info['validTo_time_t']);