Timer -settimer

HTML

<span id="timer"></span>
  <div class="coupon">Coupon</div>

CSS

.coupon{border:dashed 1px #ff0000; width: 100px; height:20px;}

JavaScript

if (sessionStorage.getItem('advertOnce') !== 'true') {
  var timestamp = parseInt(sessionStorage.getItem('timer')) || 20;
  document.getElementById("timer").innerHTML = timestamp;
  var timer = function(){
    timestamp--;
    sessionStorage.setItem('timer', timestamp);
    if(timestamp<1){
      clearInterval(counter);
      document.querySelector('.coupon').remove()
      sessionStorage.setItem('advertOnce', 'true');
    }
    document.getElementById("timer").innerHTML = timestamp;
  }
  var counter = setInterval(timer, 1000)
}