public function getPasswordDate($password = null)
    {
        if ($password === null) {
            return $this->password_set_on;
        } else {
            foreach ($this->userUsedPasswords as $usedPassword) {
                if ($usedPassword->verifyPassword($password)) {
                    return $usedPassword->set_on;
                }
            }
        }

        return null;
    }