What is the correct format for RSS feed pubdate?

Solved:

$pubDate = $article[creation_date]; 
$pubDate= date("D, d M Y H:i:s T", strtotime($pubDate));

then in my echo'd code:

 <pubDate>'.$pubDate.'</pubDate>

The PHP date function has already a way to format pubDate (RFC 2822) compliant dates:

date('r', $timestamp);