BoerseFFm.user.js

// ==UserScript==
// @id             BoerseFFm
// @name           BoerseFFm
// @namespace      tests
// @description    checking the time changing
// @include        http://www.boerse-frankfurt.de/DE/index.aspx?pageID=1
// ==/UserScript==

var clock = document.getElementById("clock");
var h, m, s, time;
var RgEx = /(\d+)\:(\d+)\:(\d+).*/;

if (!clock) return;

setInterval(function() {
  RgEx.exec(clock.textContent);
  h = RegExp.$1;
  m = RegExp.$2;
  s = RegExp.$3;
  time = h + ":" + m + ":" + s;

  if (s%30 !== 0) return;

  GM_setClipboard("Time>" + time);
  GM_notification("Time>" + time);
  GM_log(" text=" + clock.textContent + " >" + time + "<");
}, 1E3);