code: public DateMidnight minusYears(int years) {
          if (years == 0) {
              return this;
          }
          long instant = getChronology().years().subtract(getMillis(), years);
          return withMillis(instant);
      }
docs: Returns a copy of this date minus the specified number of years.
  <p>
  This datetime instance is immutable and unaffected by this method call.
  <p>
  The following three lines are identical in effect:
  <pre>
  DateTime subtracted = dt.minusYears(6);
  DateTime subtracted = dt.minus(Period.years(6));
  DateTime subtracted = dt.withFieldAdded(DurationFieldType.years(), -6);
  </pre>
  
  @param years  the amount of years to subtract, may be negative
  @return the new datetime minus the increased years
  @since 1.1