Add onclick event programmatically

But keep in mind that addEventListener is supported in IE just from version 9. To support older versions of IE you could use something like that:

if (element1.addEventListener) {  // all browsers except IE before version 9
  element1.addEventListener("click", CalCal, false);
} else {
  if (element1.attachEvent) {   // IE before version 9
    element1.attachEvent("click", CalCal);
  }
}