post.rb

class Post
  def year
    self.published_at.month
  end
  
  def month
    self.published_at.month
  end
  
  def to_params
    self.slug  
  end
end

routes.rb

Application.routes.draw do
  get '/:year/:month/:id', controller: :posts, action: :show, constraints: { year: /\d+/, month: /\d+/ }, as: "post" 
end

view.html.erb

post_path(post)