JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html> 
<html>
	<head>
		<meta charset="utf-8">
		
		<script>
			var t = new Date();
			var y = t.getFullYear();
			var m = t.getMonth() + 1;
			var d1 = t.getDate();
			var d2 = t.getDay();
		
			function displayCalender(i)
			{
			 d1 += i;
			 d2 = (d2 + i + 7) % 7;
			 var s = '<h2 align="center">' + y + '年' + m + '月</h2>';
			 s += '<h1 align="center">' + d1 + '</h1>';
			 s += '<h3 align="center">星期' + d2 + '</h3>';
			 document.getElementById("showCalender").innerHTML = s;
			}
		</script>
	</head>
	<body>
		<div id="showCalender"></div>
		<script>displayCalender(0)</script>
		<a href="javascript:displayCalender(-1)" style="float:left;">前一天</a>
		<a href="javascript:displayCalender(1)" style="float:right;">後一天</a>
	</body>
</html>