code: @Nullable
    public static GregorianCalendar getGregorianCalendar (@Nullable final XMLGregorianCalendar aCal)
    {
      if (aCal == null)
        return null;
      return aCal.toGregorianCalendar (aCal.getTimeZone (aCal.getTimezone ()),
                                       Locale.getDefault (Locale.Category.FORMAT),
                                       null);
    }
docs: Convert the passed {@link XMLGregorianCalendar} to a
  {@link GregorianCalendar}.
  
  @param aCal
  Source calendar. May be <code>null</code>.
  @return <code>null</code> if the parameter is <code>null</code>.