Are you referring to posts or topics?
Last posts are in chronological order within a forum. Topics within a forum may appear to be out of order in the sense that a topic that was started on December 13, 2009 might be listed above a topic that was started last week, but the 2009 topic has a newer post than last week's topic. For example, the topic in Open Carry called Open Carry Sightings, which was started Mar 19, 2010, is currently higher in the topic listing than American Idol Open Carry Sighting...., started May 23, 2012, because Open Carry Sightings has a newer post than the other.
You have the option of clicking the New Posts button at the top of the main index and that will bring up all new posts on the board since your last visit. But, to the best of my knowledge, there's no way to sort topics within a forum by topic opening date.
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