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