website/app/static/js/index.js

// Import Bootstrap
import 'bootstrap'
import 'bootstrap/dist/css/bootstrap.min.css'

// Import own css
import '../css/index.css'

// Import SimpleMDE
import SimpleMDE from 'simplemde'
import 'simplemde/dist/simplemde.min.css'

if (document.querySelector('textarea') > 0) {
  new SimpleMDE()
}

// Javascript
function countdown() {
  let element = document.querySelector(".countdown")
  let date = element.getAttribute('date');
  let days, hours, minutes, seconds;
  
  let endDate = new Date(date).getTime();
  if (isNaN(endDate)) {
    return;
  }

  calculate()
  setInterval(calculate, 1000);
  
  function calculate() {
    let startDate = new Date();
    startDate = startDate.getTime();
    
    let timeRemaining = parseInt((endDate - startDate) / 1000);
    
    if (timeRemaining >= 0) {
      hours = parseInt(timeRemaining / 3600);
      timeRemaining = (timeRemaining % 3600);
      
      minutes = parseInt(timeRemaining / 60);
      timeRemaining = (timeRemaining % 60);
      
      seconds = parseInt(timeRemaining);
      
      document.querySelector(".countdown .hours").innerHTML = ("0" + hours).slice(-2);
      document.querySelector(".countdown .minutes").innerHTML = ("0" + minutes).slice(-2);
      document.querySelector(".countdown .seconds").innerHTML = ("0" + seconds).slice(-2);
    } else {
      return;
    }
  }
}

window.onload = function () {
  countdown();
}