I have no idea why this isn't working but maybe a new set of eyes can help. I want to only collect posts from yesterday:
Post.find( :all, :conditions => "posts.created_at > #{(1.day.ago).to_i}", :order => "posts.created_at DESC", :include => :person)
Let Rails convert the time into the format your database expects...
:conditions => ["posts.created_at > ?", 1.day.ago]
Otherwise you're going to need to format that time so your database understands it.