static after(start, duration) {
    const dur = Duration.fromDurationLike(duration),
      dt = friendlyDateTime(start);
    return Interval.fromDateTimes(dt, dt.plus(dur));
  }
Create an Interval from a start DateTime and a Duration to extend to.
@param {DateTime|Date|Object} start
@param {Duration|Object|number} duration - the length of the Interval.
@return {Interval}