I wrote some code that required an XMLGregorianCalendar so when I poked around with code completion, the first thing I found was a constructor on
com.sun.org.apache.xerces.internal.jaxp.datatype.XMLGregorianCalendarImpl. This got my code working but when you build with Maven, you get given a warning (and rightly so):
XMLGregorianCalendarImpl is internal proprietary API and may be removed in a future release
The correct way to get something that conforms to the XMLGregorianCalendar interface is to call this factory:
XMLGregorianCalendar cal = javax.xml.datatype.DatatypeFactory.newInstance().newXMLGregorianCalendar();
Credit to the SO answer at http://stackoverflow.com/a/5402007/1410035 for how to do it.