WPMU function get_blog_details, last_updated/registered and formatting time and date string

$time = get_blog_details($blog_ID)->registered;
$unix_time = strtotime($time);

$my_time_format = date('Y-m-d H:i:s', $time);  // eg. 2011-09-28 00:20:34
$diff_format = human_time_diff($time);          // eg. 2 days

See date() and replace Y-m-d H:i:s with the format you want