isDayjs() & moment

Source Code: https://codesandbox.io/s/zen-brook-4b1rg?file=/src/index.js

```js
import dayjs2 from 'dayjs';
import moment from 'moment';

// include with script tag
const dayjs = window.dayjs;

// issue 1
// expect true
// log false
console.log(dayjs2.isDayjs(dayjs()));

// issue 2
// expect 2021-05-21 11:22:33
// log 2021-05-21 00:00:00
console.log(moment(dayjs('2021-05-21 11:22:33')).format('YYYY-MM-DD HH:mm:ss'));

```

Start with the CodeSandbox reproduction and its src/index.js entry point. Check both reported cases: isDayjs() with the script-tag global and Moment formatting of a Day.js value. Done means the two logged results match the stated expectations and the behavior is covered by repository tests.

Written by the indexing model from the issue text.