document.getElementByTagName XML

I have a handler which returns me a XML Request as an alert() with the following

<?xml version="1.0" encoding="UTF-8"?>
  <gml:TimePeriod xmlns:gml="http://ift.tt/WaETtx">
  <gml:beginPosition>2011-10-18T15:15:00.000+02:00</gml:beginPosition>
  <gml:endPosition>2014-11-23T14:45:00.000+01:00</gml:endPosition>
  </gml:TimePeriod>

The hander looks like that:

function handler(request) {
  alert(request.responseText);  
};

Now I would like parse the xml tag “beginPosition” to an array. I used the following code within the handler:

function handler(request) {
  alert(request.responseText);  
  var xmlDoc = request.responseXML;

  timeArray = xmlDoc.getElementByTagName('beginPosition').value= new Date(request.responseText).toTimeString();
  console.log(timeArray);
};

var timeArray;
alert(timeArray);

Finally I got the error message: TypeError: xmlDoc.getElementByTagName is not a function

How can I parse the date value in that array from the xml request?