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