public function setExpirationDate($date)
    {
        if (is_int($date) || ctype_digit(strval($date))) {
            $expirationDate = intval($date);
        } else {
            $expirationDate = strtotime($date);
        }

        if ($expirationDate === false) {
            throw new InvalidArgumentException(
                sprintf(
                    'Invalid expiration date format: "%s". Please use a ' .
                    'format compatible with PHP\'s strtotime().',
                    $date
                )
            );
        }

        if ($expirationDate !== 0 && $expirationDate < time() + 86400) {
            throw new InvalidArgumentException(
                'Expiration date must be at least a day in the future.'
            );
        }

        // GnuPG suffers from the 2038 bug
        if ($expirationDate > 2147483647) {
            throw new InvalidArgumentException(
                'Expiration date must not be greater than 2038-01-19T03:14:07.'
            );
        }

        $this->expirationDate = $expirationDate;

        return $this;
    }