Currently, the News Rss provider does not render the publication date of the item in the RSS feed but rather used to notify the Rss readers that this is the date when the content was published. So when you subscribe to your Rss from any reader, it would show you the publication date. This is the reason why we did not add this date to the item's content.
Anyway, if you really need this date to be shown, you have to override the news RssChannelProvider and its GetRssItems method. Once you implement the new logic, you should register the newly created class in the web.config through the Rss -> channelProviders section in the following way:
<add name="News Rss Modified Provider" type="YourType, YourAssembly" settingsControl="Telerik.News.WebControl.Rss.RssSettingsControl, Telerik.News" viewSettingsControl="Telerik.News.WebControl.Rss.RssViewControl, Telerik.News" />
Note that you should specify the control which will be shown when you set up your feed (settingsControl) and the control which would show the result once you save the feed (viewSettingsControl). In case you want to allow more options to be specified by the user, you can override the settings and view controls as well.
Hope this helps.
All the best,
the Telerik team