.add(1, 'month) also changes the day, not just the month

[henriqueleite42]

Describe the bug Day is changed to the first day of the month

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

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

dayjs.tz.setDefault('UTC');

dayjs.tz('2024-01-15').add(1, 'month').toISOString()

// Expected: '2024-02-15T00:00:00.000Z'
// Actual result: '2024-02-01T00:00:00.000Z'

Information

• Day.js Version 1.11.10
• OS: Ubuntu 22.04.3 LTS
• Browser Node 20
• Time zone: GMT-03:00, but should use UTC

[henriqueleite42]

Sorry, my mistake, I was seeing the wrong result

[henriqueleite42]

Actually, I don't believe that it's my mistake, the bug is really happening.

[buza-me]

You have a comma in your example. I wonder if you have add() defined somewhere. I just tested it out of curiosity in Node 20 on Windows and it works as expected.

[henriqueleite42]

Sorry, the comma was a typo that only exists in this example, but not in the real scenario where the bug happens.

[thadeucity]

The bug is real and it happens for .add(1, 'year') too

const initial = dayjs.tz('2024-02-15').format('YYYY-MM-DD') // 2024-02-15
const expected= dayjs('2024-02-15').add(1, 'year').format('YYYY-MM-DD') // 2025-02-15
const result = dayjs.tz('2024-02-15').add(1, 'year').format('YYYY-MM-DD') // 2025-02-01

The bug only affects the day

EDIT: The bug only happens when timezone is set to UTC EDIT2: Looks like it is a problem with: .endOf('month') when dealing with UTC timezone EDIT3: I believe it has something to do with the fact that UTC does not returns an offset (since it is ZERO), I reproduced the same error with Europe/Lisbon that has a ZERO offset too