An user reported that there's no convenience method from XMLGregorianCalendar to java.sql.Timestamp. Given that Timestamp is a JDK class, perhaps we could define one more helper method like this?

public Timestamp toTimestamp() {
  // default implementation for backward compatibility.
  // implementation can override this if it wants to make it faster.
  return new Timestamp(toGregorianCalendar().getTimeInMillis());
}