toISODate({ format = "extended", precision = "day" } = {}) {
    if (!this.isValid) {
      return null;
    }
    return toISODate(this, format === "extended", normalizeUnit(precision));
  }
Returns an ISO 8601-compliant string representation of this DateTime's date component
@param {Object} opts - options
@param {string} [opts.format='extended'] - choose between the basic and extended format
@param {string} [opts.precision='day'] - truncate output to desired precision: 'years', 'months', or 'days'.
@example DateTime.utc(1982, 5, 25).toISODate() //=> '1982-05-25'
@example DateTime.utc(1982, 5, 25).toISODate({ format: 'basic' }) //=> '19820525'
@example DateTime.utc(1982, 5, 25).toISODate({ precision: 'month' }) //=> '1982-05'
@return {string|null}