JSFiddle - React, Tailwind, and code Playground

HTML

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

JavaScript

$(function(){
			var num = 19,
					output = $('#mydiv'),
					LS_allowed=('localStorage' in window),
					restart=1/60/60*10;//Convert 1 hour to 10 seconds
			
			if(LS_allowed){
				var save_time=parseInt(localStorage.getItem('timer-time'))||0;
				if(save_time+restart*60*60*1000>=(new Date()).getTime())
					num=localStorage.getItem('timer-num')||num;
			};
			
			output.text(num);
			(function changeNum(){
				setTimeout(function(){
						if(num<=0) 
							return;
						
						output.text(--num);
						if(LS_allowed){
							localStorage.setItem('timer-num',num);
							localStorage.setItem('timer-time',(new Date()).getTime());
						};
						changeNum();
				},1000+(19-num)*100);
			})();
});