JSFiddle - React, Tailwind, and code Playground

HTML

<span data-timestamp="1320821800"></span>

JavaScript

jQuery('span[data-timestamp]').each(function(){
    var el = jQuery(this);
    var then = Math.floor(el.attr('data-timestamp'));
    var updateTime = function(){
        var now = Math.floor((new Date()) / 1000);
        el.html((now - then) + ' seconds ago');
    };
    setInterval(updateTime, 1000);
    updateTime();
});