I get updated date in wrong format using atom feed helper

Hi! I'm new in RoR and I'm using the atom feed helper to generate an atom feed in my web page but when I try to validate it using the Feed Validation service of W3C, I get the following error:

line 6, column 41: updated must be an RFC-3339 date-time: Thu Apr 17 22:05:00 +0200 2008

It seems the plugin doesnt return the updated date for the whole feed in the right format and I dont see the way to change it.The rest of the feed looks fine: the published and updated node inside each entry node is in the right format.

Maybe there si something wrong with the version of the plugin? do I have to updated it? Is there any way to fix it?

Thanks a lot,