code: public LocalDate atEndOfQuarter() {
          Month month = quarter.firstMonth().plus(2);
          return LocalDate.of(year, month, month.maxLength());
      }
docs: Returns a {@code LocalDate} at the end of the quarter.
  <p>
  This returns a {@code LocalDate} based on this year-quarter.
  The day-of-quarter is set to the last valid day of the quarter, taking
  into account leap years.
  <p>
  This method can be used as part of a chain to produce a date:
  <pre>
  LocalDate date = year.atQuarter(quarter).atEndOfQuarter();
  </pre>
  
  @return the last valid date of this year-quarter, not null