Unable to parse a date

HEAD

```

dayjs.extend(window.dayjs_plugin_customParseFormat);
```

SCRIPT:

```
const dateString = dayjs("29-09-2021", "DD-MM-YYYY").format('YYYY-MM-DD');
console.log("Date: ", dateString);
```

RESULT

Date: Invalid Date

Start by running the issue's inline reproduction with the customParseFormat plugin and verify the Invalid Date result. Trace the date-string parsing path used by the plugin, then add or update a regression test for "29-09-2021" with "DD-MM-YYYY". Done means the formatted result is "2021-09-29" without breaking existing parsing tests.

Written by the indexing model from the issue text.