Invalid date error when manipulating dates with timezone plugin on Firefox and Safari

**Describe the bug**
I'm getting an `Invalid date` error when I try to manipulate timezoned dates on Firefox and Safari. It works as expected on Chrome. I am using `startOf`, but it seems that other manipulative methods are giving invalid dates as well.

**Expected behavior**
I'm expecting a valid date on all browsers.

**Information**
- Day.js 10.0.4
- Browsers: Chrome 89 (😎), Firefox 87 (🙁), Safari 14 (🙁)

**Minimum code to reproduce it:**

```
import dayjs from "dayjs";
import utc from "dayjs/plugin/utc";
import timezone from "dayjs/plugin/timezone";

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

const date = dayjs().tz("America/New_York").startOf("month");

console.log(date.format());
```

Start with the provided reproduction using the utc and timezone plugin entry points, then compare startOf("month") in Chrome, Firefox, and Safari. Trace the timezone date manipulation path and add a regression test for the browser failure. Done means the reproduction produces a valid, matching formatted date in all three browsers.

Written by the indexing model from the issue text.