JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Stopwatch</h1>
<p>Time(milliseconds): <span id="time">0</span></p>
<input type="button" id="startstop" value="Start" />

CSS

body {
    font: 100%/40px helvetica, arial, sans-serif;   
}

h1 {
    font-size: 30px;   
}

JavaScript

var start = false;
var d1;
var d2;
var d3;
var currentTime;
var endTime;
var realtimeInterval;

$('#startstop').click(function(){
    $this = $(this);
    if(start === false){
        realtimeCount();
        d1 = new Date();
        currentTime = d1.getTime();
        start = true;
        $this.val('Stop');        
    }
    else {        
        d2 = new Date();
        endTime = d2.getTime();
        stopwatchTime = endTime-currentTime;
        clearInterval(realtimeInterval);
        $('#time').html(stopwatchTime);
        start = false;
        $this.val('Start');        
    }
});

var realtimeCount = function(){
    realtimeInterval = setInterval(function(){
        d3 = new Date();
        realtimeCurrent = d3.getTime();
        realtimeVal = realtimeCurrent-currentTime;
        $('#time').text(realtimeVal);
    }, 1);
}