startOf not working when the timezone is set to UTC+0:00

**Describe the bug**
When I set the timezone to Africa/Accra(UTC+00:00), the startOf function starts behaving weirdly.

Code
```
import './App.css';
import dayjs from 'dayjs';
import timezone from 'dayjs/plugin/timezone';
import utc from 'dayjs/plugin/utc';
import { useState } from 'react';

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

function App() {
const renderTime = (tz) => {
const inputDate = dayjs("2023-07-10 09:00:00").tz("Asia/Calcutta")
dayjs.tz.setDefault(tz)
const now = dayjs.tz(inputDate)
const date = now.format();
const startDate = now.startOf('d').format();
return {
date,
startDate
}
};

const {date, startDate} = renderTime('Africa/Accra')
const {date: date1, startDate: startDate1} = renderTime('Asia/Calcutta')

return (

export default App;
```

**Expected behavior**
The Start time in case of 'Africa/Accra' should be 2023-07-10T00:00:00Z and not 2023-07-09T18:30:00Z

**Information**
- Day.js Version 1.11.7
- OS: 13.4.1
- Browser: Version 114.0.5735.198 (Official Build) (arm64)
- Time zone: GMT+05:30

Start by reproducing the provided renderTime example with the timezone and utc plugins, comparing Africa/Accra and Asia/Calcutta. Trace the startOf('d') behavior through the timezone-related entry points and add a regression test for the expected Africa/Accra midnight result. Done means the reproduced case returns 2023-07-10T00:00:00Z without breaking the other timezone case.

Written by the indexing model from the issue text.