JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block1">Блок 1</div>
<div class="block2">Блок 2</div>

CSS

* {
    margin: 0;
    padding: 0;
}
html, body {
    width: 100%;
    height: 100%;
}
div {
    width: 100px;
    height: 100px;
    position: absolute;
    color: white;
    text-align: center;
    line-height: 100px;
}
.block1 {
    top: 0;
    left: 0;
    background: red;
}
.block2 {
    bottom: 0;
    right: 0;
    background: blue;
}

JavaScript

var block1 = document.querySelector(".block1");
var block2 = document.querySelector(".block2");
var startTime, endTime, result;


block1.onmouseout = function()
{
    startTime = new Date();
};
block2.onclick = function()
{
    endTime = new Date();
    result = endTime - startTime;
    alert(result + " миллисекунд");
};