Is it possible to parse only valid Date strings according to ISO8601

It is possible to check whether string according ISO8601 is a valid date?

For example `dayjs('1970-00-00', 'YYYY-MM-DD', true).isValid()` should return `false` in my opinion, because because month 0 or day 0 do not exist.

Providing all possible date formats according ISO8601 is cumbersome and would look like this

```
dayjs('2021-12-01T00:00:01', [
'YYYY', 'YYYY-MM', 'YYYY-MM-DD',
'YYYY-MM-DDTHH', 'YYYY-MM-DDTHH:mm', 'YYYY-MM-DDTHH:mm:ss', 'YYYY-MM-DDTHH:mm:ss.S', 'YYYY-MM-DDTHH:mm:ss.SS', 'YYYY-MM-DDTHH:mm:ss.SSS',
'YYYY-MM-DDTHHZ', 'YYYY-MM-DDTHH:mmZ', 'YYYY-MM-DDTHH:mm:ssZ', 'YYYY-MM-DDTHH:mm:ss.SZ', 'YYYY-MM-DDTHH:mm:ss.SSZ', 'YYYY-MM-DDTHH:mm:ss.SSSZ',
'YYYY-MM-DDTHHZZ', 'YYYY-MM-DDTHH:mmZZ', 'YYYY-MM-DDTHH:mm:ssZZ', 'YYYY-MM-DDTHH:mm:ss.SZZ', 'YYYY-MM-DDTHH:mm:ss.SSZZ', 'YYYY-MM-DDTHH:mm:ss.SSSZZ'
], true).isValid()

```

However, time zone can be provided as UTC offset, e.g, `+02:30`, `+02`, `+0230`, `+2` but also as `Z` indicating UTC time. The [List of all available parsing tokens](https://day.js.org/docs/en/parse/string-format#list-of-all-available-parsing-tokens) apparently does not support `Z` for UTC.

Can you add proper support for ISO8601 date formats?

Wernfried

Start by reviewing Day.js's strict parsing behavior and the ISO8601 formats and offsets listed in the issue. Compare the current handling of month/day zero, Z, and UTC offsets against the requested formats; done means valid ISO8601 strings parse correctly and invalid dates are rejected.

Written by the indexing model from the issue text.