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