Jekyll::Hooks.register :pages, :pre_render do |page|
  begin
    # get the current post last modified time
    modification_time = File.mtime( page.path )
    
    # inject modification_time in post's datas.
    page.data['last-modified-date'] = modification_time
  rescue
    page.data['last-modified-date'] = "1970-01-01"
  end
end