how to get time from server in javascript code example

Example: get time from server and run it from there in javascript

var serverTime = 1490856278000;
    var expected = serverTime;
    var date;
    var h;
    var m;
    var s;
    var now = performance.now();
    var then = now;
    var dt = 0;
    var nextInterval = (interval = 1000);

    setTimeout(step, interval);
    function step() {
      then = now;
      now = performance.now();
      dt = now - then - nextInterval;
      nextInterval = interval - dt;
      serverTime += interval;
      date = new Date(serverTime);
      h = date.getHours();
      m = date.getMinutes();
      s = date.getSeconds();

      var session = "AM";

      if (h == 0) {
        h = 12;
      }

      if (h > 12) {
        h = h - 12;
        session = "PM";
      }

      h = h < 10 ? "0" + h : h;
      m = m < 10 ? "0" + m : m;
      s = s < 10 ? "0" + s : s;

      var time = h + ":" + m + ":" + s + " " + session;

      document.getElementById("NowTime").innerHTML = time;

      now = performance.now();

      setTimeout(step, Math.max(0, nextInterval));
    }