I'm sure the solution is that your oldDateString is something like "2009-10-20". Obviously this does not contain any time data lower than days. If you format this string with your new formatter where should it get the minutes, seconds and milliseconds from?
So the result is absolutely correct: 2009-10-20 00:00:00.000
What you'll need to solve this, is the original timestamp (incl. time data) before your first formatting.