system/HTTP/DownloadResponse.php
public function setLastModified($date)
	{
		if ($date instanceof \DateTime)
		{
			$date->setTimezone(new \DateTimeZone('UTC'));
			$this->setHeader('Last-Modified', $date->format('D, d M Y H:i:s') . ' GMT');
		}
		elseif (is_string($date))
		{
			$this->setHeader('Last-Modified', $date);
		}

		return $this;
	}
public function setLastModified($date)
	{
		if ($date instanceof \DateTime)
		{
			$date->setTimezone(new \DateTimeZone('UTC'));
			$this->setHeader('Last-Modified', $date->format('D, d M Y H:i:s') . ' GMT');
		}
		elseif (is_string($date))
		{
			$this->setHeader('Last-Modified', $date);
		}

		return $this;
	}
{@inheritDoc}