public function getEntropy()
    {
        if ($this->year < 100) {
            // two-digit year
            $entropy = $this->log(self::NUM_DAYS * self::NUM_MONTHS * 100);
        } else {
            // four-digit year
            $entropy = $this->log(self::NUM_DAYS * self::NUM_MONTHS * self::NUM_YEARS);
        }
        // add two bits for separator selection [/,-,.,etc]
        if (!empty($this->separator)) {
            $entropy += 2;
        }

        return $entropy;
    }