toISODate() {
    if (!this.isValid) return INVALID;
    return `${this.s.toISODate()}/${this.e.toISODate()}`;
  }
Returns an ISO 8601-compliant string representation of date of this Interval.
The time components are ignored.
@see https://en.wikipedia.org/wiki/ISO_8601#Time_intervals
@return {string}