Exibe um elemento após N segundos - Facebook - https://www.facebook.com/groups/frontendbrasil/permalink/3977507682297954/

showDelay.js

const seconds = 30;

const lsKey = "acesso-anterior";

if (localStorage.getItem(lsKey) === null)
  setTimeout(showElements, seconds * 1000);
else
  showElements();
  
function showElements() {
  localStorage.setItem(lsKey, new Date());
  
  const elems = document.querySelector(".classeAtualASerRemovido");

  for (var i = 0; i < elems.length; i++)
    elems[i].className = "classeParaExibir";
}