public function setCreatedAt($createdAt)
    {
        if ($createdAt instanceof \DateTime
            || (is_object($createdAt) && method_exists($createdAt, 'format'))) {
            $this->createdAt = $createdAt->format('Y-m-d H:i:s');
        } else {
            $this->createdAt = $createdAt;
        }
    }