plus(duration) {
    if (!this.isValid) return this;

    const dur = Duration.fromDurationLike(duration),
      result = {};

    for (const k of orderedUnits) {
      if (hasOwnProperty(dur.values, k) || hasOwnProperty(this.values, k)) {
        result[k] = dur.get(k) + this.get(k);
      }
    }

    return clone(this, { values: result }, true);
  }
Make this Duration longer by the specified amount. Return a newly-constructed Duration.
@param {Duration|Object|number} duration - The amount to add. Either a Luxon Duration, a number of milliseconds, the object argument to Duration.fromObject()
@return {Duration}