Dayjs.diff appears to malfunction when the dates have different time zones

**Description**

When one of the dates has been altered with `tz`, calling `date.diff(otherDate, "day")` produces unexpected results:

```js
const dayjs = require("dayjs")
const utc = require("dayjs/plugin/utc")
const timezone = require("dayjs/plugin/timezone")
dayjs.extend(utc)
dayjs.extend(timezone)

const now = dayjs()

const works = now.add(121, "hour")
works.diff(now, "day") // 5 ✅

const doesntWork = now.tz("America/Chicago").add(121, "hour")
doesntWork.diff(now, "day") // 4 ❌

works.valueOf() === doesntWork.valueOf() // true
works.toISOString() === doesntWork.toISOString() // true
works.diff(now, "hour") === doesntWork.diff(now, "hour") // true (both 121)
```

**Expected behavior**

Since the dates are the same internally whether `tz` was called or not, `diff` should produce identical results.

**Information**
- Day.js version: 1.11.0
- OS: Ubuntu 20.04
- Node: 14.18.3

Start with the minimal JavaScript reproduction using the utc and timezone plugins, then trace the diff and tz entry points. Compare day and hour results for the two equivalent dates; done means diff returns identical day results when valueOf() and toISOString() match.

Written by the indexing model from the issue text.