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