doc = Nokogiri::HTML::DocumentFragment.parse(html_content)

doc.css("p.pubdate.online").each do |node|
   node.content = self.send("#{article_type}_released_at").strftime("%B %d, %Y")
 end

doc.to_html

doc = Nokogiri::HTML(html_content)

node = doc.at_css("p.pubdate.online")
node.content = self.send("#{article_type}_released_at").strftime("%B %d, %Y")
doc.to_html