Incorrect .add() result with timezone plugin

[YiJing233]

Describe the bug
Hi Team, there is a confusing problem I'm facing with:
There is a timestamp, and I transform it into a dayjs object with the timezone plugin. But when I use .add(1, 'month'), the result is wrong, which seems to be added too much😫.

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

const originalDate = dayjs.tz(1669852800000, "Etc/GMT");
console.log("originalDate", originalDate);
console.log('formated OriginalDate', originalDate.toISOString());
const addedDate = originalDate.add(1, "month");
console.log("addedDate", addedDate);
console.log('formated addedDate', addedDate.toISOString());

And the result is:

originalDate M {$L: "en", $d: Thu Dec 01 2022 00:00:00 GMT+0800 (中国标准时间), $x: Object, $y: 2022, $M: 11…}

formated OriginalDate 2022-12-01T00:00:00.000Z 

addedDate  M {$L: "en", $u: true, $offset: 0, $d: Tue Jan 31 2023 00:00:00 GMT+0800 (中国标准时间), $x: Object…}

formated addedDate 2023-01-31T00:00:00.000Z 

Here is my codesandbox link: https://codesandbox.io/s/dank-lake-kc5w1i?file=/src/index.js

Expected behavior
It should be '2022-01-01T00:00:00.000Z'

Information

• Day.js Version v1.11.7
• OS: MacOS Monterey 12.5.1
• Browser Chrome 108.0.5359.124 (x86_64)
• Time zone: [Asia/Shanghai / UTC +8]

[KronosDev-Pro]

I suppose this is a GMT topic is in issue #2037 and solved in pull request #2118.

Test

describe('issue 2195 - add in UTC timezone', () => {
  it('Thu Dec 01 2022 00:00:00 GMT+0800, with "+ 1 month"', () => {
    dayjs.extend(utc)
    dayjs.extend(timezone)

    expect(dayjs(1669852800000).tz(GMT).add(1, 'month').format()).toBe('2023-01-01T00:00:00Z')
    expect(dayjs(1669852800000).tz(GMT).add(1, 'month').isSame(moment(1669852800000).tz(GMT).add(1, 'M'))).toBe(true)
  })
})
// pass

result : 2023-01-01T00:00:00.000Z

[YiJing233]

Sorry that I can not get the same result as you did 😭. I cloned the repo and installed moment-timezone as Moment.js Doc said and then run this case, but the output of testing is failed.

// timezone.test.js
import MockDate from 'mockdate'
// import moment from 'moment'
import dayjs from '../src'
import timezone from '../src/plugin/timezone'
import utc from '../src/plugin/utc'
import moment from 'moment-timezone';
// ...
it('test issue #2195', () => {
  console.log(dayjs(1669852800000).tz('GMT').add(1, 'month'))
  expect(dayjs(1669852800000).tz('GMT').add(1, 'month').isSame(moment(1669852800000).tz('GMT').add(1, 'M'))).toBe(true);
})

And the result is

    Expected value to be:
      true
    Received:
      false
 Dayjs  object '$d': 2023-01-30T11:00:00.000Z,
 Moment Moment<2023-01-01T00:00:00Z>

Moment is the correct result.

I have debugged the running progress of .add() function, and I noticed that It happened when somewhere (maybe .clone()?) setting the $d value with .getUTCDate() which seems got '30',

[KronosDev-Pro]

You cloned this repo https://github.com/BePo65/dayjs/tree/fix/issue2037 ?

[KronosDev-Pro]

I updated the test above

here is the feedback:

[YiJing233]

I cloned the repo itself dayjs and run the test cases in branch 'dev', here is my result:

And I found that when my machine timezone setting was UTC+8, the cases failed, but when I changed to UTC+0, cases passsed.

[KronosDev-Pro]

Okay, have you tried with this repo dayjs ? it fixes some timezone plugin errors

[KronosDev-Pro]

I have set the time zone on my machine to UTC+8.

With the dayjs repo, I have the same error
while on the 'fix/issue2037' repo, I have no errors

[YiJing233]

Ok, I used the repo you said above and changed the branch to 'fix/issue2037' and the test case passed successfully.🎉

Thanks for your patience, and may I ask when will this branch be merged into main?

[KronosDev-Pro]

no worries, with pleasure.

unfortunately I don't have this information.

[KronosDev-Pro]

Hi 👋
@YiJing233 don't forget to close this issue, if is solve 😉

[zxd65885152]

Hi,
Same problem suffered.
I'd like to ask whether the pr is merged?

[KronosDev-Pro]

Hi 👋🏼
@zxd65885152 Unfortunately the fix has still not been merged and no sign of a possible merge on the horizon