There is an error in class org.geotools.xml.xsi.Date in method getValue(Element, ElementValue, Attributes, Map).
Calendar class is used to retrieve a Date from String. In Calendar months are indexed from 0 but this is not taken into account by GeoTools.
If we get in the XML first of January: 2009-01-01 it is parsed to a date object 2009-02-01. January is indexed 0 in Calendar class, 1 is February.