How to use the moment-timezone.parseZone function in moment-timezone

function formatDTSTART(string, tz) {
    
    if (!string) {
      return 'DTSTART:' + moment().format('YYYYMMDD');
    }

    if (tz) {
      // Form #3: DATE WITH LOCAL TIME AND TIME ZONE REFERENCE
      return 'DTSTART;TZID=' + tz + ':' + moment(string).format('YYYYMMDDTHHmm00');
    }

    if (isDateTime(string) && moment.parseZone(string).utcOffset() === 0 && !isUTC(string)) {
      // Form #1: DATE WITH LOCAL TIME
      return 'DTSTART:' + moment(string).format('YYYYMMDDTHHmm00');
    }

    if (isDateTime(string)) {
      // Form #2: DATE WITH UTC TIME
      return 'DTSTART:' + moment.utc(string).format('YYYYMMDDTHHmm00') + 'Z';
    }

    return 'DTSTART;VALUE=DATE:' + moment(string).format('YYYYMMDD');
  }

    activity = _.sortBy( activity, a => ( moment.parseZone( a.created_at ) ) );
    // attempting to match the logic in the computervision/score_observation endpoint

const date = (...args) => {
  let d;
  if (args.length === 1) {
    const arg = args[0];
    if (typeof arg === 'string') {
      try {
        d = arg === '' ? moment.parseZone(UTCTimePart, time_ISO_8601) : parseDate(arg);
      } catch (err) {
        throw err;
      }
    } else if (typeof arg === 'object') {
      if (arg instanceof Date) {
        const ISO = arg.toISOString();
        const dateTime = moment.parseZone(ISO);
        const datePart = dateTime.format(date_ISO_8601);
        d = moment.parseZone(`${datePart}${UTCTimePart}`);
      } else if (arg.isDateTime) {
        const dateTime = moment.tz(arg.format(), UTC);
        const datePart = dateTime.format(date_ISO_8601);
        d = moment.parseZone(`${datePart}${UTCTimePart}`);
      }
      if (!d.isValid()) {
        throw new Error('Invalid date. Parsing error while attempting to create date from date and time');
      }
    } else {
      throw new Error('Invalid format encountered. Please specify date in one of these formats :\n- "date("2012-12-25")"\n- date_and_time object');
    }
  } else if (args.length === 3 && isNumber(args)) {
    const [year, month, day] = args;
    d = moment.tz({ year, month, day, hour: 0, minute: 0, second: 0 }, UTC);

const valueDT = (obj) => {
  const e = moment.parseZone(epoch, date_ISO_8601);
  const duration = moment.duration(obj.diff(e));
  return duration.asSeconds();
};

const parseDate = (str) => {
  try {
    const d = moment.parseZone(`${str}${UTCTimePart}`);
    if (d.isValid()) {
      return d;
    }
    throw new Error('Invalid date. This is usually caused by an inappropriate format. Please check the input format.');
  } catch (err) {
    return err;
  }
};

taxon={taxon}
            url={noTaxonLink ? null : `/taxa/${taxon.id}`}
            noParens
            target={linkTarget}
            user={config.currentUser}
            showMemberGroup
          />
        
        { item.body && (  ) }
      
    );
  } else {
    header = I18n.t( "user_commented", { user: ReactDOMServer.renderToString( userLink ) } );
    contents = (  );
  }
  const relativeTime = moment.parseZone( item.created_at ).fromNow( );
  let panelClass;
  const headerItems = [];
  const unresolvedFlags = _.filter( item.flags || [], f => !f.resolved );
  if ( unresolvedFlags.length > 0 ) {
    panelClass = "flagged";
    headerItems.push(
      <span>
        <a rel="nofollow noopener noreferrer" href="{`/${isID">
          <i>
          { " " }
          { I18n.t( "flagged_" ) }
        </i></a></span>

const valueInverseDT = (value, offset = 'Z') => {
  const e = moment.parseZone(epoch, date_ISO_8601);
  return dateAndTime(e.add(value, 'seconds').utcOffset(offset).format());
};

onChangeDate = (value) => {
    const yyyymmdd = moment.parseZone(value).format('YYYY-MM-DD');
    const timezone = toTimezone(this.props.timezone || this.props.orgTimezone);
    const z = moment(yyyymmdd).tz(timezone).format('Z');
    this.onChangeCriterion(this.getOperator(), `${yyyymmdd}T00:00${z}`);
  };

const date = (...args) => {
  let d;
  if (args.length === 1) {
    const arg = args[0];
    if (typeof arg === 'string') {
      try {
        d = arg === '' ? moment.parseZone(UTCTimePart, time_ISO_8601) : parseDate(arg);
      } catch (err) {
        throw err;
      }
    } else if (typeof arg === 'object') {
      if (arg instanceof Date) {
        const ISO = arg.toISOString();
        const dateTime = moment.parseZone(ISO);
        const datePart = dateTime.format(date_ISO_8601);
        d = moment.parseZone(`${datePart}${UTCTimePart}`);
      } else if (arg.isDateTime) {
        const dateTime = moment.tz(arg.format(), UTC);
        const datePart = dateTime.format(date_ISO_8601);
        d = moment.parseZone(`${datePart}${UTCTimePart}`);
      }
      if (!d.isValid()) {
        throw new Error('Invalid date. Parsing error while attempting to create date from date and time');
      }
    } else {
      throw new Error('Invalid format encountered. Please specify date in one of these formats :\n- "date("2012-12-25")"\n- date_and_time object');
    }
  } else if (args.length === 3 && isNumber(args)) {
    const [year, month, day] = args;
    d = moment.tz({ year, month, day, hour: 0, minute: 0, second: 0 }, UTC);
    if (!d.isValid()) {
      throw new Error('Invalid date. Parsing error while attempting to create date from parts');