Weblog RSS feeds are not updating immediately when new posts are made. There is a delay of several minutes and in some instances, several hours.
The RSS feed updates instantly as you add a new entry. The News Reader part is typically intended for external RSS sources and for performance reasons, it caches the feed for an hour or so before it refreshes. Post Date and Time fields are not only for sorting / listing purpose, they are also for when the story is published for users to view.