userscript.js

// ==UserScript==
// @name         highlight new items
// @match        https://www.reddit.com/r/*/
// @icon         https://www.google.com/s2/favicons?sz=64&domain=reddit.com
// ==/UserScript==

(function() {
    'use strict';

    let hours = 12;
    let now = new Date();
    let items = document.querySelectorAll('.thing:not(.promoted)');

    for (let item of items) {
        let anchor = item.querySelector('.title a');
        let time = item.querySelector('time');
        let delta = now - new Date(time.dateTime);

        if (delta < hours*60*60*1000) {
            anchor.style.background = 'yellow';
        }
    }
})();