You can't. The site outputs the feed and decides what it allows you to pull from it. It's not going to be outputting stuff from a month ago - it's only going to be outputting the latest x items or so.
Sometimes you can search the site for other qualifiers you can add to the end of the feed url to maybe bring up other stuff that isn't in the main feed. (Like how you can add stuff to the end of a vbulletin feed url.) You'll have to look them up on the site though, if they even post them/have them.
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