FeedWriter\Feed::setDate PHP Method

    public function setDate($date)
    {
        if ($this->version == Feed::RSS1) {
            throw new InvalidOperationException('The publication date is not supported in RSS1 feeds.');
        }
        // The feeds have different date formats.
        $format = $this->version == Feed::ATOM ? \DATE_ATOM : \DATE_RSS;
        if ($date instanceof DateTime) {
            $date = $date->format($format);
        } else {
            if (is_numeric($date) && $date >= 0) {
                $date = date($format, $date);
            } else {
                if (is_string($date)) {
                    $timestamp = strtotime($date);
                    if ($timestamp === FALSE) {
                        throw new \InvalidArgumentException('The given date was not parseable.');
                    }
                    $date = date($format, $timestamp);
                } else {
                    throw new \InvalidArgumentException('The given date is not an instance of DateTime, a UNIX timestamp or a date string.');
                }
            }
        }
        if ($this->version == Feed::ATOM) {
            $this->setChannelElement('updated', $date);
        } else {
            $this->setChannelElement('lastBuildDate', $date);
        }
        return $this;
    }