RSS feed parameters

Hi,

I was wondering whether there were parameters for the RSS feeds that could limit items fetched from a feed by time period. e.g.: fetch items from feed that were posted in the last X months from now? I've found http://scn.sap.com/docs/DOC-29261 but only lists the full and numItems parameters, which are not suitable. I need a parameter that limits by time period.

Thoughts?