code: public DateMidnight minusDays(int days) {
          if (days == 0) {
              return this;
          }
          long instant = getChronology().days().subtract(getMillis(), days);
          return withMillis(instant);
      }
docs: Returns a copy of this date minus the specified number of days.
  <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.minusDays(6);
  DateMidnight subtracted = dt.minus(Period.days(6));
  DateMidnight subtracted = dt.withFieldAdded(DurationFieldType.days(), -6);
  </pre>
  
  @param days  the amount of days to subtract, may be negative
  @return the new datetime minus the increased days
  @since 1.1