JSFiddle - React, Tailwind, and code Playground

HTML

<label for="dt1">Start Date : </label><input type="date" id="dt1">
<label for="dt2">End Date : </label><input type="date" id="dt2" onfocusout="validateDate()">

JavaScript

function validateDate() {
	var d1 = new Date.parse(document.getElementsByName("dt1").value);
	var d2 = new Date.parse(document.getElementsByName("dt2").value);
	
	if (d1.getTime() > d2.getTime())
		alert("Start date cannot be greater then End date");

}