public static boolean isExpired(final X509CRL crl, final ZonedDateTime reference) {
        return reference.isAfter(DateTimeUtils.zonedDateTimeOf(crl.getNextUpdate()));
    }
Determines whether the given CRL is expired by comparing the nextUpdate field
with a given date.

@param crl       CRL to examine.
@param reference Reference date for comparison.
@return True if reference date is after CRL next update, false otherwise.
core/cas-server-core-util-api/src/main/java/org/apereo/cas/util/crypto/CertUtils.java