Countdown timer in pure JavaScript

countdown.js

function countdown(endDate) {
  let days, hours, minutes, seconds;
  
  endDate = new Date(endDate).getTime();
  
  if (isNaN(endDate)) {
	return;
  }
  
  setInterval(calculate, 1000);
  
  function calculate() {
    let startDate = new Date().getTime();
    
    let timeRemaining = parseInt((endDate - startDate) / 1000);
    
    if (timeRemaining >= 0) {
      days = parseInt(timeRemaining / 86400);
      timeRemaining = (timeRemaining % 86400);
      
      hours = parseInt(timeRemaining / 3600);
      timeRemaining = (timeRemaining % 3600);
      
      minutes = parseInt(timeRemaining / 60);
      timeRemaining = (timeRemaining % 60);
      
      seconds = parseInt(timeRemaining);
      
      document.getElementById("days").innerHTML = parseInt(days, 10);
      document.getElementById("hours").innerHTML = hours < 10 ? "0" + hours : hours;
      document.getElementById("minutes").innerHTML = minutes < 10 ? "0" + minutes : minutes;
      document.getElementById("seconds").innerHTML = seconds < 10 ? "0" + seconds : seconds;
    } else {
      return;
    }
  }
}

(function () { 
  countdown('04/01/2333 05:00:00 PM'); 
}());

@oyenmwen, glad it helps! :)