Hi Jaime, try to use a Date object. An example:
var oTwoDaysAgo = new Date();
oTwoDaysAgo.setDate(oTwoDaysAgo.getDate() - 2);
var oFinanceArticle = new sap.suite.ui.commons.FeedItem({
title: "America's best little beach towns", image: "images/grass.jpg", link: "http://www.cnn.com/2013/06/12/travel/best-beach-towns/index.html", source: "CNN.com - Top Stories", publicationDate: oTwoDaysAgo
});Pau