Countdown with padded seconds

countdown.js

// set the date we're counting down to
var target_date = new Date("Aug 15, 2019").getTime();
 
// variables for time units
var days, hours, minutes, seconds;
 
// get tag element
var countdown = document.getElementById("countdown");
 
// update the tag with id "countdown" every 1 second
setInterval(function () {
 
    // find the amount of "seconds" between now and target
    var current_date = new Date().getTime();
    var seconds_left = (target_date - current_date) / 1000;
 
    // do some time calculations
    days = parseInt(seconds_left / 86400);
    seconds_left = seconds_left % 86400;
     
    hours = parseInt(seconds_left / 3600);
    seconds_left = seconds_left % 3600;
     
    minutes = parseInt(seconds_left / 60);
    seconds = parseInt(seconds_left % 60);
    
    //NEW CODE
    var secondsString = seconds.toString();
    
    // format countdown string + set tag value
    countdown.innerHTML = days + "d, " + hours + "h, "
    + minutes + "m, " + (secondsString.length > 1 ? secondsString : "0" + secondsString) + "s";  
 
}, 1000);