Timezone conversion incorrect when daylight saving in effect

**Describe the bug**
Dayjs does not seem to convert timezone correctly when DST is in effect. It produces different results from Moment. The Moment results seem intuitively what I'd expect, so believe to be correct unless there is some subtlety I'm missing.

**Expected behavior**
With this code:
```javascript
const when = '2020-07-30T12:00:00+00:00'; // Midday GMT in summer
console.log('Moment London: ' + moment(when).tz('Europe/London').format('HH:mm Z'));
console.log('Moment New York: ' + moment(when).tz('America/New_York').format('HH:mm Z'));

console.log('Dayjs London: ' + dayjs(when).tz('Europe/London').format('HH:mm Z'));
console.log('Dayjs New York: ' + dayjs(when).tz('America/New_York').format('HH:mm Z'));
```

The output is:
```javascript
Moment London: 13:00 +01:00
Moment New York: 08:00 -04:00
Dayjs London: 12:00 +00:00 // Expect 13:00 +01:00
Dayjs New York: 08:00 -05:00 // Expect 08:00 -04:00
```

**Information**
- Day.js: v1.9.6
- OS: Windows 10 Home
- Nodejs: v12.8.3
- Time zone: GMT+00:00

The issue names no source file or test; begin by running the supplied JavaScript reproduction with the timezone conversion entry point and compare Day.js with Moment for London and New York. Done means DST-aware output matches the stated expectations: 13:00 +01:00 for London and 08:00 -04:00 for New York.

Written by the indexing model from the issue text.