src/Helper/JWT.php
protected function expires(DateTimeImmutable $now, string $expires = null)
    {
        return !$expires ? null : $now->modify($expires)->getTimestamp();
    }
protected function expires(DateTimeImmutable $now, string $expires = null)
    {
        return !$expires ? null : $now->modify($expires)->getTimestamp();
    }
Verify expiration contraint and return the timestamp of expiration date.
If null will return null too and token will never expire

@param DateTimeImmutable $now
@param string|null $expires
@return int|null