code: public static Years<CalendarUnit> between(CalendarMonth m1, CalendarMonth m2) {
  
          int delta = m1.getYear() - m2.getYear();
  
          if (delta > 0) {
              if (m2.getMonth().compareTo(m1.getMonth()) < 0) {
                  delta--;
              }
          } else if (delta < 0) {
              if (m2.getMonth().compareTo(m1.getMonth()) > 0) {
                  delta++;
              }
          }
  
          return Years.ofGregorian(delta);
  
      }
docs: /*[deutsch]
  <p>Bestimmt die Jahresdifferenz zwischen den angegebenen Kalendermonaten. </p>
  
  @param   m1  first calendar month
  @param   m2  second calendar month
  @return  year difference