Javascript countdown timer for the difference between two dates

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let timer = setInterval(function() {

  // The dispatch date is just the variable name. You can change it wo whatever. As long as you change it where ever else it is been used as well.  
  // the format for dispatch_date is ('2018-10-17 17:00:00') which is been pull from an input field with the id of "countdown_timer"
  const dispatch_date = new Date(document.getElementById("countdown_timer").value);
  const today = new Date();
  
  // console.log("dispatch_date : "+dispatch_date);
  // console.log("today : "+today);

  // get the difference
  const diff = dispatch_date - today;

  // math
  let days = Math.floor(diff / (1000 * 60 * 60 * 24));
  let hours = Math.floor((diff % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
  let minutes = Math.floor((diff % (1000 * 60 * 60)) / (1000 * 60));
  let seconds = Math.floor((diff % (1000 * 60)) / 1000);

  // display
  document.getElementById("timer").innerHTML =

  //Days - Hours - Minutes - Seconds
    "</div class='timer-p'>This Stock list Expires in </div> \ <div class=\"days\"> \
  <div class=\"numbers\">" + days + "</div>Days</div> \
<div class=\"hours\"> \
  <div class=\"numbers\">" + hours + "</div>Hours</div> \
<div class=\"minutes\"> \
  <div class=\"numbers\">" + minutes + "</div>Minutes</div> \
<div class=\"seconds\"> \
  <div class=\"numbers\">" + seconds + "</div>Seconds</div> \
</div>";

}, 1000);