dayjs.tz(time, timezone), when time contains time zone, the result is incorrect

[willingfox2]

Describe the bug
Example: dayjs.tz('2024-04-03T16:21:18+08:00', "America/New_York").format()
Desired result：2024-04-03T04:21:18-04:00
Actual result: 2024-04-03T08:21:18-04:00

Since New York is in the fourth borough in the west, which is 12 hours different from the eighth borough in the east, the result should be 2024-04-03T04:21:18-04:00, but it is not actually like this.

[michaelhillebrand]

I am having the same issue essentially. I am setting a default timezone and then parsing without the second parameter.

See code example below

const dayjs = require('dayjs');
const utc = require('dayjs/plugin/utc');
const timezone = require('dayjs/plugin/timezone');

dayjs.extend(utc);
dayjs.extend(timezone);

dayjs.tz.setDefault('America/New_York');
const timestamp = '2024-05-17T10:00:00-08:00';

const test1 = dayjs.tz(timestamp);
console.log('test1.format', test1.format());
// expect  2024-05-17T14:00:00-04:00
// getting 2024-05-17T18:00:00-04:00
console.log('test1.toString', test1.toString());
// expect  Fri, 17 May 2024 18:00:00 GMT
// getting Fri, 17 May 2024 22:00:00 GMT

// This works, but I don't want to have to convert after I parse every time
const test2 = dayjs(timestamp).tz();

console.log('test2.format', test2.format());
// get  2024-05-17T14:00:00-04:00
console.log('test2.toString', test2.toString());
// get  Fri, 17 May 2024 18:00:00 GMT

[IonelLupu]

I just encountered this issue as well. Here is a playground that replicates the issue: https://codesandbox.io/p/sandbox/dayjs-playground-forked-dy3xwn?file=%2Fsrc%2Findex.js%3A4%2C1

import dayjs from "dayjs";
import timezonePlugin from "dayjs/plugin/timezone";
import utc from "dayjs/plugin/utc";

dayjs.extend(utc);
dayjs.extend(timezonePlugin);

dayjs.tz.setDefault("Asia/Seoul");

console.log(
  dayjs("2024-07-02T00:00:00Z").format("ddd DD MMM YYYY"),
  "default timezone (which should be Asia/Seoul) -> wrong dat"
);

console.log(
  dayjs("2024-07-02T00:00:00Z").tz("Asia/Seoul").format("ddd DD MMM YYYY"),
  "manual timezone (Asia/Seoul) -> correct date"
);

Basically, dayjs.tz.setDefault("Asia/Seoul"); doesn't do anything. We have to manually set the timezone for each of our dates