Converting timestamp to date

I am trying to convert a timestamp in milliseconds to a new Date.

If I use this code, it returns Fri Aug 09 2013 20:00:00 GMT-0400 (Eastern Daylight Time):

x = 1376092800000;
y = new Date(x);
document.write(y);

But when I change

x = record[i].getElementsByTagName("date_of_consent")[0].childNodes[0].nodeValue;

The document returns "Invalid Date".

I have an XML file with timestamps that I need to convert to date objects. Any suggestions on ways to handle this conversion? Thanks.