function countdown(targetDate) {
  var nowMillis = new Date().getTime();
  var targetMillis = targetDate.getTime();
  var diff = targetMillis - nowMillis;
  var years = Math.floor(diff / 3.154e+10);
  var diffMinusYears = diff - (years * 3.154e+10);
  var months = Math.floor(diff / 2.628e+9) % 12;
  var diffMinusMonths = diffMinusYears - (months * 2.628e+9);
  var days = Math.floor(diffMinusMonths / 8.64e+7);
  var hours = Math.floor(diff / 3.6e+6 ) % 24;
  var mins = Math.floor(diff / 60000 ) % 60;
  var secs = Math.floor(diff / 1000 ) % 60;

  return [ years, months, days, hours, mins, secs ];
}

var target = new Date(2020, 0, 31, 0, 0);

window.setInterval(function() {
  console.log(countdown(target));
}, 1000);