change time base on timezone momen-timezone

i want to convert selected time into timezone saved in user profile using moment-timezone library

my local system timezone is PKT(pakistan) and user has saved timezone in his Profile is AEST(australian/sydney). so when user select time i just want that time in userTimezone and send in API.
e.g

const time  =  "07:00"
const date = "06/13/2023"
const timezone = "Australian/Sydney"
moment(`${date} ${time}`).tz(timezone).format() // 2023-06-20T12:00:00+10:00

converted time should be 7Am AESt but library has convert it into 12pm. if i change moment default timezone into user profile timezone then still i dont get correct output

e.g

const time  =  "07:00"
const date = "06/13/2023"
const timezone = "Australian/Sydney"
 moment.tz.setDefault(timezone)

 moment(`${date} ${time}`).tz(timezone).format() //2023-06-20T12:00:00+10:00
 moment(`${date} ${time}`).format() //2023-06-20T12:00:00+10:00
moment.tz(`${date} ${time}`, timezone).format() // 2023-06-20T17:00:00+10:00

you can see none of the output is what i am trying to acheive. it would be greate if you let me know other library if moment does not support this.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

// use the following npm package, "luxon"
// https://moment.github.io/luxon/api-docs/index.html

const { DateTime } = require("luxon");

const yourInputTime = "13:45:21";
const yourDateInput = "2023-06-25";
// these inputs are in your timezone

// now you want to convert these to let's day Singapore timezone
// you can find the whole list of timezone from the following link
/* https://stackoverflow.com/questions/38399465/how-to-get-list-of-all-timezones-in-javascript/54500197#54500197 */

// first construct an ISO string
const yourInputISO = `${yourDateInput}T${yourInputTime}`;
const convertedISO = DateTime.fromISO(yourInputISO).setZone("Asia/Singapore").toISO(); // 2023-06-25T15:45:21.000+08:00

const yourDesiredTime = convertedISO.split("T")[1].split(".")[0] // 15:45:21

<!-- end snippet -->

Start by reading the supported values as part of the Intl Web/JS APi and checking what's available => https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/supportedValuesOf . Anything that's returned in the list you can use in your code/app and browser - but may not work in other clients and browsers.

Unless you dig deep into the topic of time I wouldn't try to convert time that a user has entered from a new location selected in their profile - this could be complicated. Better to let the user pick a location and let the location set the time then save that. Calculation back and off of the time? No need - just use Int API to read from.

That said have a try at my code using no external libraries:

1. User logs in and is based in Pakistan or wherever
2. User then changes the timezone to Singapore or somewhere else - this is done in this section of code let userSelectedCountry = 'Asia/Singapore' and call our function by using the Date and RegEx APIs to achieve what we need and pull out the correct time for selected (available timezone in our array!) timezone

To use it follow step 1 and 2 in the code comments.

    const event = new Date();
    //list of locales - NOT exhaustive
    const localeCodes = ['el', 'en-au', 'en-ca', 'en-gb', 'en-us', 'es-es', 'es-mx','it-it']
    //list of timezones - NOT exhaustive
    const timeZonesL =   ["Africa/Abidjan"
          , "Asia/Aden"
          , "Asia/Almaty"
          , "Asia/Amman"
          , "Asia/Anadyr"
          , "Asia/Aqtau"
          , "Asia/Aqtobe"
          , "Asia/Ashgabat"
          , "Asia/Ashkhabad"
          , "Asia/Atyrau"
          , "Asia/Baghdad"
          , "Asia/Bahrain"
          , "Asia/Baku"
          , "Asia/Bangkok"
          , "Asia/Barnaul"
          , "Asia/Beirut"
          , "Asia/Bishkek"
          , "Asia/Brunei"
          , "Asia/Calcutta"
          , "Asia/Chita"
          , "Asia/Choibalsan"
          , "Asia/Chongqing"
          , "Asia/Chungking"
          , "Asia/Colombo"
          , "Asia/Dacca"
          , "Asia/Damascus"
          , "Asia/Dhaka"
          , "Asia/Dili"
          , "Asia/Dubai"
          , "Asia/Dushanbe"
          , "Asia/Famagusta"
          , "Asia/Gaza"
          , "Asia/Harbin"
          , "Asia/Hebron"
          , "Asia/Ho_Chi_Minh"
          , "Asia/Hong_Kong"
          , "Asia/Hovd"
          , "Asia/Irkutsk"
          , "Asia/Istanbul"
          , "Asia/Jakarta"
          , "Asia/Jayapura"
          , "Asia/Jerusalem"
          , "Asia/Kabul"
          , "Asia/Kamchatka"
          , "Asia/Karachi"
          , "Asia/Kashgar"
          , "Asia/Kathmandu"
          , "Asia/Katmandu"
          , "Asia/Khandyga"
          , "Asia/Kolkata"
          , "Asia/Krasnoyarsk"
          , "Asia/Kuala_Lumpur"
          , "Asia/Kuching"
          , "Asia/Kuwait"
          , "Asia/Macao"
          , "Asia/Macau"
          , "Asia/Magadan"
          , "Asia/Makassar"
          , "Asia/Manila"
          , "Asia/Muscat"
          , "Asia/Nicosia"
          , "Asia/Novokuznetsk"
          , "Asia/Novosibirsk"
          , "Asia/Omsk"
          , "Asia/Oral"
          , "Asia/Phnom_Penh"
          , "Asia/Pontianak"
          , "Asia/Pyongyang"
          , "Asia/Qatar"
          , "Asia/Qostanay"
          , "Asia/Qyzylorda"
          , "Asia/Rangoon"
          , "Asia/Riyadh"
          , "Asia/Saigon"
          , "Asia/Sakhalin"
          , "Asia/Samarkand"
          , "Asia/Seoul"
          , "Asia/Shanghai"
          , "Asia/Singapore"
          , "Asia/Srednekolymsk"
          , "Asia/Taipei"
          , "Asia/Tashkent"
          , "Asia/Tbilisi"
          , "Asia/Tehran"
          , "Asia/Tel_Aviv"
          , "Asia/Thimbu"
          , "Asia/Thimphu"
          , "Asia/Tokyo"
          , "Asia/Tomsk"
          , "Asia/Ujung_Pandang"
          , "Asia/Ulaanbaatar"
          , "Asia/Ulan_Bator"
          , "Asia/Urumqi"
          , "Asia/Ust-Nera"
          , "Asia/Vientiane"
          , "Asia/Vladivostok"
          , "Asia/Yakutsk"
          , "Asia/Yangon"
          , "Asia/Yekaterinburg"
          , "Asia/Yerevan"
          , "Europe/London"]
    
    getDateTime = (country) => { 
      const regexp = new RegExp(`\\b${country}\\b`, 'gi') // showing how to pass optional flags
      const userTimeZone = timeZonesL.filter(timeZ => timeZ == timeZ.match(country))
      return event.toLocaleString('en-GB', { timeZone: userTimeZone });
      //return event.toLocaleString('en-GB', { timeZone: 'Asia/Singapore' });;
    }
    
    //step 1
    //test 1 - after user signs in and selects new country/timezone we call it for them
    //e.g. from a dropdown
    //Value in drop down must match a value in the timeZonesL array
    let userSelectedCountry = 'Asia/Singapore'
    //build regex and pass in selected country
    var re = new RegExp("\\b"+userSelectedCountry+"\\b"); 
    //we then call our function with it as a param if it tests truthy to regex match      
    console.log(getDateTime(userSelectedCountry.match(re)))
    
    //step 2
    //On sign out save the time as to what the user selected e.g. 
    //mongoDB.save(getDateTime(userSelectedCountry.match(re)))
    //now we can save any time the user selects
    //NOTE: more needs to be done to finalise list of timezones as some are redundant, not supported by some
    //browsers and you don't want all of them

i could not solve this issue with moment-timezone libarary and so i shift to dayjs it solved my problem. it is correctly converting time in selected timezone. below is the code for this

import dayjs from "dayjs";
import utc from "dayjs/plugin/utc"; // Import the UTC plugin for dayjs
import timezone from "dayjs/plugin/timezone";
import customParseFormat from "dayjs/plugin/customParseFormat";

// Extend dayjs with the required plugins
dayjs.extend(utc);
dayjs.extend(timezone);
dayjs.extend(customParseFormat);

const getStartAndEndDateTime = (date, time, timezone, duration) => {
  let startDateTime = "";
  let endDateTime = "";

  if (date && time) {
    const formateDDate = dayjs(date).format("YYYY-MM-DD");
    const formatedTime = dayjs(time, "hh:mm a").format("H:mm");
    const timestamp = `${formateDDate} ${formatedTime}`;

    startDateTime = dayjs.tz(timestamp, timezone || dayjs.tz.guess()).format();
    endDateTime = dayjs
      .tz(timestamp, timezone || dayjs.tz.guess())
      .add(duration.value, "m")
      .format();
  }
  return { start_datetime: startDateTime, end_datetime: endDateTime };
};