plus(duration) {
    if (!this.isValid) return this;
    const dur = Duration.fromDurationLike(duration);
    return clone(this, adjustTime(this, dur));
  }
Add a period of time to this DateTime and return the resulting DateTime

Adding hours, minutes, seconds, or milliseconds increases the timestamp by the right number of milliseconds. Adding days, months, or years shifts the calendar, accounting for DSTs and leap years along the way. Thus, `dt.plus({ hours: 24 })` may result in a different time than `dt.plus({ days: 1 })` if there's a DST shift in between.
@param {Duration|Object|number} duration - The amount to add. Either a Luxon Duration, a number of milliseconds, the object argument to Duration.fromObject()
@example DateTime.now().plus(123) //~> in 123 milliseconds
@example DateTime.now().plus({ minutes: 15 }) //~> in 15 minutes
@example DateTime.now().plus({ days: 1 }) //~> this time tomorrow
@example DateTime.now().plus({ days: -1 }) //~> this time yesterday
@example DateTime.now().plus({ hours: 3, minutes: 13 }) //~> in 3 hr, 13 min
@example DateTime.now().plus(Duration.fromObject({ hours: 3, minutes: 13 })) //~> in 3 hr, 13 min
@return {DateTime}