Finally, I killed all my laziness in updating the planet. By the virtue of Michael Greene script that adds correct <pubDate> 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.)