JSFiddle - React, Tailwind, and code Playground

HTML

<input id="input"/>

JavaScript

var start = 0;
$("#input").keyup(function(e) {
    if(e.keyCode == 37) {
        start = new Date().getTime();
    } else if(e.keyCode == 39) {
        var elapsed = new Date().getTime() - start;
        alert("elapsed time in milliseconds is: " + elapsed);
        // start again
        start = 0;
    }
});