Converting a local time to UTC sometimes applies one extra hour

[henhal]

Describe the bug

Converting a UTC time to a local time using timezone plugin seems to apply the difference in DST compare to the current time when converting back to JSON.
So if I have a time expressed in Europe/Stockholm DTS time, and my machine is on Stockholm standard time, when converting the time to JSON, an extra hour seems to be subtracted.

Example:
Today is 2022-02-24, I'm in Europe/Stockholm which is currently on standard time, no daylight savings, so UTC+01:00.

import dayjs from 'dayjs';
import timezone from 'dayjs/plugin/timezone';
import utc from 'dayjs/plugin/utc';

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

const winterUtc = dayjs.utc('2022-03-01T15:00:00Z');
console.log('Winter UTC JSON:', winterUtc.toJSON());
const winterLocal = winterUtc.tz('Europe/Stockholm'); // 15.00 UTC is 16:00+01:00 in March
console.log('Winter Europe/Stockholm format:', winterLocal.format());
// prints 2022-03-01T16:00:00+01:00  👍
console.log('Winter Europe/Stockholm UTC offset:', winterLocal.utcOffset());
// prints 60 👍
console.log('Winter Europe/Stockholm JSON:', winterLocal.toJSON());
// prints 2022-03-01T15:00:00.000Z 👍

const summerUtc = dayjs.utc('2022-06-01T15:00:00Z');
console.log('Summer UTC JSON:', summerUtc.toJSON());
const summerLocal = summerUtc.tz('Europe/Stockholm'); // 15:00 UTC is 17:00+02:00 in June
console.log('Summer Europe/Stockholm format:', summerLocal.format());
// prints 2022-06-01T17:00:00+02:00 👍
console.log('Summer Europe/Stockholm UTC offset:', summerLocal.utcOffset());
// prints 120 👍
console.log('Summer Europe/Stockholm JSON:', summerLocal.toJSON());
// prints 2022-06-01T14:00:00.000Z, should be 2022-06-01T15:00:00.000Z 👎

It's unclear to me if that always happens or if it's due to my local machine being on standard time while I'm computing a date in DST, and if the opposite problem would happen if my local machine is on DST while computing a "winter time" date.

Expected behavior

A time expressed in a time zone should be possible to convert to UTC / JSON by applying the UTC offset regardless of the local machine's own time zone.

Information

• Day.js Version [e.g. v1.7.0]
• OS: [e.g. MacOS 12]
• Browser: N/A, Node 14
• Time zone: Europe/Stockholm, UTC+01:00

[henhal]

I confirmed that on 1.10.4, JSON representation is correct for a localized DST date, but instead the UTC offset is wrong, it's 60 instead of 120 even during the summer, meaning the formatted time is also +01:00 instead of +02:00.
On 1.10.5, the behaviour changed to correct format() but break toJSON() for localized DST dates.

[henhal]

Even smaller example:

// returns a date representing 2022-01-01T15:00:00Z, i.e. correct
dayjs.utc(`2022-01-01T15:00:00Z`).tz('Asia/Tokyo').utc();

// returns a date representing 2022-06-01T14:00:00Z, i.e. one hour off. 
dayjs.utc(`2022-06-01T15:00:00Z`).tz('Asia/Tokyo').utc();

Note that Asia/Tokyo doesn't even have DST so why this extra hour is offset if converting a date in the summer I don't know, but I'm speculating it's because my local machine is in a time zone with DST during that period, in which case something is fishy since my local machine's time zone should not be involved at all here.