code: public LocalDate plusDays(int days) {
          if (days == 0) {
              return this;
          }
          long instant = getChronology().days().add(getLocalMillis(), days);
          return withLocalMillis(instant);
      }
docs: Returns a copy of this date plus the specified number of days.
  <p>
  This LocalDate instance is immutable and unaffected by this method call.
  <p>
  The following three lines are identical in effect:
  <pre>
  LocalDate added = dt.plusDays(6);
  LocalDate added = dt.plus(Period.days(6));
  LocalDate added = dt.withFieldAdded(DurationFieldType.days(), 6);
  </pre>
  
  @param days  the amount of days to add, may be negative
  @return the new LocalDate plus the increased days