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