code: public XMLGregorianCalendar addDays(final XMLGregorianCalendar cal, final int amount) {
          XMLGregorianCalendar to = buildXMLGregorianCalendarDate(cal);
          // Add amount of months
          to.add(addDays(amount));
          return to;
      }
docs: Add Days to a Gregorian Calendar.
  
  @param cal The XMLGregorianCalendar source
  @param amount The amount of days. Can be a negative Integer to substract.
  @return A XMLGregorianCalendar with the new Date