Finally, I killed all my laziness in updating the planet. By the virtue of Michael Greene script that adds correct to rss feed of xanga (still don't know why they use this weird format). The script can be found in here. However, xanga had fixed their malformed <channel> tag already so you have to comment out these two lines in the original Greene's code

$xml = str_replace('</channel><item>','<item>',$xml);
$xml = str_replace('</rss>','</channel></rss>',$xml);

Well, from now on, when cache failed, century ago entries of xanga's bloggers wouldn't magically show up again. (or at least, show up in the correct date.)