code: public static Years<CalendarUnit> between(CalendarQuarter q1, CalendarQuarter q2) {
  
          int delta = q2.getYear() - q1.getYear();
  
          if (delta > 0) {
              if (q2.getQuarter().compareTo(q1.getQuarter()) < 0) {
                  delta--;
              }
          } else if (delta < 0) {
              if (q2.getQuarter().compareTo(q1.getQuarter()) > 0) {
                  delta++;
              }
          }
  
          return Years.ofGregorian(delta);
  
      }
docs: /*[deutsch]
  <p>Bestimmt die Jahresdifferenz zwischen den angegebenen Quartalen. </p>
  
  @param   q1  first quarter year
  @param   q2  second quarter year
  @return  year difference