Is DateString supposed to be truncating seconds?

Question

If I configure `` to format dates as ISO dates with

$DateStringFormat := {"Year", "-", "Month", "-", "Day", "T", ":", 
      "Hour", ":", "Minute", ":", "SecondExact", 
      If[$TimeZone == 0, "Z", "UTC" <> ToString[$TimeZone]]}

I get truncated seconds, rather than rounded seconds

{2012, 10, 12, 21, 32, 22.5359} // DateString
2012-10-12T:21:32:22.535UTC-5.

This seems wrong. The result should either be

2012-10-12T:21:32:22.5359UTC-5.

or

2012-10-12T:21:32:22.536UTC-5.

Is it a bug, or am I misunderstanding "SecondExact" or DateString?