[рони]

helish_88,
https://learn.javascript.ru/datetime

<script>
  var mydate = new Date("07/31/2017");
  var date = new Date();
  date.setHours(0, 0, 0);
  var n = Math.round((date - mydate) / 864E5)
  document.write("n = " + n)
</script>