window.clockSync.clientTimeAheadOfServerInMilliseconds

gistfile1.html

<script src="~/Scripts/moment.min.js"></script>
<script>
  (function() {
    var clientNow = moment();
    var serverNow = moment('@(DateTimeOffset.UtcNow.ToString("o"))');
    var diff = clientNow.diff(serverNow);
    if (!!window.performance && !!window.performance.timing) {
      var responseStartToNow = new Date().valueOf() - window.performance.timing.responseStart;
      diff -= responseStartToNow;
    }
    window.clockSync = { clientTimeAheadOfServerInMilliseconds: diff };
  })();
</script>