You don't need to convert your dates to timestamp before the sorting, but it's a good idea though because it will take more time to sort without it.
$data = array(
array(
"title" => "Another title",
"date" => "Fri, 17 Jun 2011 08:55:57 +0200"
),
array(
"title" => "My title",
"date" => "Mon, 16 Jun 2010 06:55:57 +0200"
)
);
function sortFunction( $a, $b ) {
return strtotime($a["date"]) - strtotime($b["date"]);
}
usort($data, "sortFunction");
var_dump($data);