Adding a month to a date removes an hour.

**Describe the bug**
Adding a month to a date removes an hour.

**Expected behavior**
```
const initialDate = dayjs('2022-02-28T00:00:00.000Z');
const oneMonthAdded = initialDate.add(1, 'month');
console.log(oneMonthAdded.toISOString());
```
Should output `2022-03-28T00:00:00.000Z`

**Actual behaviour**
Outputs `2022-03-27T23:00:00.000Z`

**Information**
- Day.js Version: 1.10.7
- OS: Win 10
- Browser: N/A
- Time zone: GMT+2

Run the provided Day.js reproduction in the reported GMT+2 environment and compare its output with the expected ISO timestamp. Trace the implementation reached by add(1, 'month') and its date/time-zone handling; the fix is done when the example preserves 00:00:00.000Z and relevant behavior is covered by a regression test.

Written by the indexing model from the issue text.