Is there any best practice for handling timezone in Js and Firebase?

my problem:
I have 1 computer that sends data (also timestamp) to some firebase collection, let׳s say – Las Vegas, Nevada.

When Im trying to fetch data on my firebase functions I get the Timestamp property in GMT.

and my firebase function׳s propose is to return to frontend all Timestamps occur on specific day of the week, eventually I should get 7 days of the week with timestamps on each day.

my problem is to get GMT time and return the time based on clients location/timezone
on each timestamp from the collection so I can show it on a graph on the client side.

the status now: is I get timestamps that occur on Sunday, but I know that nothing happend on Sunday for sure.

no matter what I have tried, nothing helps.. I will be glad for some help 🙂
code below.

/////util functions:
const exposeDayName = (date) => {
  return new Intl.DateTimeFormat('en-US', { weekday: 'long' }).format(date);
}

const fillMissingDays = (countObject) => {
  const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
  const today = days.indexOf(exposeDayName(new Date()));
  const fullWeekCount = {};

  for (let i = today + 1; i < (days.length + today + 1); i++) {
    const indexDay = i < 7 ? i : i - 7;
    fullWeekCount[days[indexDay]] = countObject[days[indexDay]] ? countObject[days[indexDay]] : 0;
  }
  return fullWeekCount;
}
/////

const getAllWeeklySessionsDividedByDays = async (req, res) => {
  try {
    //function i have tried to solve this problem with no success.
    function convertTZ(date, tzString) {
      return new Date((typeof date === "string" ? new Date(date) : date).toLocaleString("en-US", { timeZone: tzString }));

    }

    let timezone = req.body.timezone;
    let id = req.body.id;

    const currentDate = new Date();
    const previousWeekDate = new Date(currentDate.getTime() - 6 * 24 * 60 * 60 * 1000);

    const conversationRef = db.collection("conversation");
    const sessionsArr = [];

    const snapshot = await conversationRef
      .where('id', '==', Id)
      .where("timestamp", ">=", previousWeekDate)
      .where("timestamp", "<=", currentDate)
      .get();

    snapshot.forEach(doc => {
      const element = doc.data();
      sessionsArr.push({
        Id: element.id,
        sessionId: element.session_id,
        timestamp: exposeDayName(element.timestamp.toDate())
      });
    });

    const uniqueSessionsArr = [...new Map(sessionsArr.map(v => [JSON.stringify(v), v])).values()];

    const countSessionsPerDay = {};
    uniqueSessionsArr.forEach(session => {
      const day = session.timestamp;
      countSessionsPerDay[day] = countSessionsPerDay[day] ? countSessionsPerDay[day] + 1 : 1;
    });

    const fullWeekSessionsPerDay = fillMissingDays(countSessionsPerDay);

    const result = {
      labels: Object.keys(fullWeekSessionsPerDay),
      datasets: [
        {
          label: "Daily Sessions",
          data: Object.values(fullWeekSessionsPerDay)
        },
      ],
    }

    res.status(200).send(result);
  } catch (err) {
    res.status(400).send(err.message);
  }
}