Improvement of resolvePageDate

[Mister-Hope]

Currently, resolvePageDate is returning 1970-01-01 when the Date is empty or invaid. And the date is use to built permalinks.

https://github.com/vuepress/vuepress-next/blob/55f780c4a516b2c0119328874906647af80ac73b/packages/%40vuepress/core/__tests__/page/resolvePageDate.spec.ts#L27-L66

And in that case, if set premalink with date globally, we will get links like below with /1970/01/01/foo-bar/frontmatter .

https://github.com/vuepress/vuepress-next/blob/55f780c4a516b2c0119328874906647af80ac73b/packages/%40vuepress/core/__tests__/page/resolvePagePermalink.spec.ts#L48-L61

I think that's not good enough, visitors would be confused if they are not familiar with coding.

My ideas is to let them fill with __(or x - _ or something else). so that if global permalinks with date are set, those without valid date would be set as /__/__/__/foo-bar/. I think this can be much more clear to site owners or visitors to release that it's not having date. 😀

---

Also I would suggest date without day like below return with 2020-10-__

https://github.com/vuepress/vuepress-next/blob/55f780c4a516b2c0119328874906647af80ac73b/packages/%40vuepress/core/__tests__/page/resolvePageDate.spec.ts#L77-L86

So that we can built permalinks like /2020/10/__/foo-bar/, looking forward to your opinion.

[meteorlxy]

That looks so weird.

And will complicate the logic

[Mister-Hope]

This parts of code changing is just affecting permalinks now, and at least in V1 it's not used in other places. So I think some special handling is ok.😑

[meteorlxy]

What about 0000-00-00 ?

[Mister-Hope]

I think it's better, let me ask my friend about it

[Mister-Hope]

3 of them replied and agreed, this won't let them confuse if it's a acticle that do write in 1970.😀

[meteorlxy]

Change to 0000-00-00 in v2.0.0-alpha.24