set({ start, end } = {}) {
    if (!this.isValid) return this;
    return Interval.fromDateTimes(start || this.s, end || this.e);
  }
"Sets" the start and/or end dates. Returns a newly-constructed Interval.
@param {Object} values - the values to set
@param {DateTime} values.start - the starting DateTime
@param {DateTime} values.end - the ending DateTime
@return {Interval}