JSFiddle - React, Tailwind, and code Playground

HTML

<div id="r"></div>

JavaScript

//window.onload=function(){
var start=Date.now(),r=document.getElementById('r');
(function f(){
 var diff=Date.now()-start,ns=(((3e5-diff)/1000)>>0),m=(ns/60)>>0,s=ns-m*60;
 r.textContent="Registration closes in "+m+':'+((''+s).length>1?'':'0')+s+' minutes';
 if(diff>(3e5)){start=Date.now()}
 setTimeout(f,1000);
})();
//}