get time from server and run it from there in javascript

1var serverTime = 1490856278000;
2    var expected = serverTime;
3    var date;
4    var h;
5    var m;
6    var s;
7    var now = performance.now();
8    var then = now;
9    var dt = 0;
10    var nextInterval = (interval = 1000);
11
12    setTimeout(step, interval);
13    function step() {
14      then = now;
15      now = performance.now();
16      dt = now - then - nextInterval;
17      nextInterval = interval - dt;
18      serverTime += interval;
19      date = new Date(serverTime);
20      h = date.getHours();
21      m = date.getMinutes();
22      s = date.getSeconds();
23
24      var session = "AM";
25
26      if (h == 0) {
27        h = 12;
28      }
29
30      if (h > 12) {
31        h = h - 12;
32        session = "PM";
33      }
34
35      h = h < 10 ? "0" + h : h;
36      m = m < 10 ? "0" + m : m;
37      s = s < 10 ? "0" + s : s;
38
39      var time = h + ":" + m + ":" + s + " " + session;
40
41      document.getElementById("NowTime").innerHTML = time;
42
43      now = performance.now();
44
45      setTimeout(step, Math.max(0, nextInterval));
46    }