The gx:TimeStamp is supported by Google Earth 6. If you're using an older version than 6.2.2 then you should upgrade to 6.2.2 and the most current 7.x version.
You can download 6.2.2 or the latest 7.1.1 version here.
http://support.google.com/earth/bin/answer.py?hl=en&answer=168344
Check out the full answer posted here: