@Override
    public Stream<T> entries() {
        return backingMap.values().stream();
    }
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