Simple countdown timer

Trying to get to the simplest way to countdown to a specific date

by ian_smithz

HTML

<div id="countdown"></div>

JavaScript

//Global variables
 var monthNames = [ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" ];

//Countdown function
function countdown (){
    //Get today's date
    today = new Date();
    
    //Setup the final date in UTC time
    BigDay = new Date(Date.UTC(2018,04,03,23,59));
    
    //Calculate the time left from now till the final date
    timeLeft = (BigDay.getTime() - today.getTime());
    
    daysLeft = Math.floor(timeLeft / (24 * 60 * 60 * 1000));
    hrsLeft = Math.floor(timeLeft / (1000*60*60)%24);
    minsLeft = Math.floor(timeLeft / (1000*60)%60);
    secsLeft= Math.floor(timeLeft / 1000)%60;
    
    //What to display in the browser
    document.getElementById('countdown').innerHTML = daysLeft + " days " + hrsLeft +" hours " + minsLeft + " minutes and "+secsLeft+" seconds left";
    //Show the final date
    document.getElementById('countdown').innerHTML += "<br>until "+monthNames[BigDay.getUTCMonth()] + " " + BigDay.getUTCDate() + " " + BigDay.getUTCFullYear() + " "+ BigDay.getUTCHours() + ":" + BigDay.getUTCMinutes();
    //Show the final date in local time
    document.getElementById('countdown').innerHTML += "<br>which is "+monthNames[BigDay.getMonth()] + " " + BigDay.getDate() + " " + BigDay.getFullYear() + " "+ BigDay.getHours() + ":" + BigDay.getMinutes()+" local time";
}

//Refresh the countdown function every second
setInterval(countdown, 1000);