SC Datetime format in Project Service

The various date fields in the project service seem to require a Datetime format of the following form: 2009-04-08T00:00:00-05:00
The fields don't appear to store anything but the date, but the service connect update method still fails if we don't use the format listed above. We have been testing just appending T00:00:00-05:00 to the end of the required date, but on 2 out of about 15 samples Vantage converted the date to 1 day earlier. I am assuming the -5:00 is the time offset. Can anyone help me understand why it would have bumped the dates up by one day? It only did it on 2 of about 30 dates we tested. One date is 2010-01-30, and the other is 2009-11-05. I don't see anything significant about those dates.
Thanks,
Craig