Thanks Diogo, glad you’re finding the software useful!
About your question, the item_date parameter was added for our Feed Creator tool - so it’s intended to be used for sites which don’t already offer a feed. Here’s an example of how it works (have a look at the ‘Submitted parameters’ section):
It has to be used in combination with the item parameter. The item parameter is used to tell Feed Creator which elements contain the information (URL, title, date, description) of each news item. The item_title, item_date, item_url, item_desc parameters are then applied within the context of the item element to select and extract these elements.
However, your question suggests you’re using Full-Text RSS, not Feed Creator, on a site that already has a feed. In this case, the original date in the original feed should be preserved by Full-Text RSS. At least it is if I try putting http://www.engadget.com/rss.xml through Full-Text RSS.
If you’re working with a feed that doesn’t include item dates. You can tell Full-Text RSS to extract this from the source article when it’s pulling out the body. To do that, you need to create or edit the site config file associated with the site and tell it which element holds the date value. See http://help.fivefilters.org/customer/portal/articles/223153-site-patterns for more information on how to do this.
Hope that’s some help.