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