Date-related Error from feed

And here is the format in the xml

<pubDate>Fri, 19 Apr 2013 10:20:56 +0300</pubDate>

I tried fixing the error by changing the code into

case "publishdate":
		$pub_date = get_post_meta( get_the_ID(), 'wprss_item_date', true );
		if(is_string($pub_date)){
			$pub_date = (int) $pub_date;
		}
                $publishdate = date( 'Y-m-d H:i:s', $pub_date) ;
                echo $publishdate;
                break;

However while there is no error anymore, the echoed date is always 1970-01-01 00:00:00 which probably due to the unexpected format of the rss date.