JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mydiv"></div>

JavaScript

var timer,
		def=19,
    num = ('localStorage' in window)?(localStorage.getItem('timer-num')||def):def,
    output = $('#mydiv');

function changeNum(){
    output.text(num--);
		if('localStorage' in window)
				localStorage.setItem('timer-num',num);
    //каждый раз задержка будет на 100ms больше
    if(num >= 0) setTimeout(changeNum,500+(19-num)*100);
}
setTimeout(changeNum,500);