Timezone plugin different output

**Describe the bug**
```
dayjs.tz('2018-09-17T05:38:46.000-07:00', 'America/Los_Angeles').format('MM/DD/YYYY [at] h:mm A z')
"09/17/2018 at 12:38 PM PDT"
```
```
dayjs('2018-09-17T05:38:46.000-07:00').tz('America/Los_Angeles').format('MM/DD/YYYY [at] h:mm A z')
"09/17/2018 at 5:38 AM PDT"
```

```
moment.tz('2018-09-17T05:38:46.000-07:00', 'America/Los_Angeles').format('MM/DD/YYYY [at] h:mm A z')
"09/17/2018 at 5:38 AM PDT"
```

is this expected behavior?

**Information**
- Day.js Version 1.9.1
- OS: macOS 10.15.17
- Browser chrome 85.0.4183.121
- Time zone: GMT+3

Start by comparing the two documented dayjs timezone calls with the Moment result and inspect the timezone plugin's parsing and conversion behavior. Confirm which output is expected for the offset-bearing input, then verify the chosen behavior against the reproduced formatting example and a regression test if the repository provides one.

Written by the indexing model from the issue text.