private static double submissionRate(final ZonedDateTime a, final ZonedDateTime b) {
        val rate = SUBMISSION_RATE_DIVIDEND / (a.toInstant().toEpochMilli() - b.toInstant().toEpochMilli());
        LOGGER.debug("Submission rate between [{}] and [{}] is [{}]", a, b, rate);
        return rate;
    }
Computes the instantaneous rate in between two given dates corresponding to two submissions.

@param a First date.
@param b Second date.
@return Instantaneous submission rate in submissions/sec, e.g. {@code a - b}.
core/cas-server-core-authentication-throttle/src/main/java/org/apereo/cas/throttle/BaseMappableThrottledSubmissionsStore.java