code: public static int birthdayCompare (@Nullable final LocalDate aDate1, @Nullable final LocalDate aDate2)
    {
      if (EqualsHelper.identityEqual (aDate1, aDate2))
        return 0;
      if (aDate1 == null)
        return -1;
      if (aDate2 == null)
        return 1;
  
      // first compare month
      int ret = aDate1.getMonth ().compareTo (aDate2.getMonth ());
      if (ret == 0)
      {
        // on equal month, compare day of month
        ret = aDate1.getDayOfMonth () - aDate2.getDayOfMonth ();
      }
      return ret;
    }
docs: Compare two dates by birthday. This means, the dates are only compared by
  day and month, and <b>not</b> by year!
  
  @param aDate1
  First date. May be <code>null</code>.
  @param aDate2
  Second date. May be <code>null</code>.
  @return same as {@link Comparator#compare(Object, Object)}