Serialized dates.. what are extra number?

Hi,
I have a simple webservice that has some date fields like the following
[WebMethod]
public MyClass Function1()
{
  MyClass _MyClass = new MyClass();
  _MyClass.Date1 = //get_date_from_db.ToUniversalTime();
  return _MyClass;
}
Now when I call the webservice from the browser, I see the date in the following format
2009-06-03T04:03:59Z -- which is all good
But some dates are in the following format
2009-06-17T21:37:33.173Z -- What is this number after the period at the end?
I need to remove this number and the period and make all dates like the first format. I can't use any ToString() formatting because I am just assiging the date to a DateTime field. or can I?
Any ideas would be appreciated!
Thanks
...