Date not added to post frontmatter

[travis-w]

• I confirm that this is an issue rather than a question.

Bug report

I have been messing around with VuePress the last couple days along with the blog plugin. I am working on creating my own theme and noticed the blogs were being "sorted" oldest to newest and I wanted to reverse it. I found the sorter function and console.logged the prev.frontmatter.date and next.frontmatter.date and both are undefined. It doesn't look like the dates are getting added to the frontmatter of posts at all.

Steps to reproduce

• I prepared a reproduction repo, here is the reproduction repo:
  https://github.com/travis-w/vuepress-bug

• This issue doesn't need a reproduction repro, here is the steps to reproduce

What is expected?

• Newest blog (according to dates in filename) should show first.
• Dates for blogs should be logged in console (from sorter in /.vuepress/config.js)

What is actually happening?

• Oldest blog is showing first
• undefined being logged in console for dates in frontmater

Other relevant information

• npx vuepress info:
  Environment Info:

  System:
  OS: macOS 10.15.7
  CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
  Binaries:
  Node: 14.2.0 - ~/.nvm/versions/node/v14.2.0/bin/node
  Yarn: Not Found
  npm: 7.5.3 - ~/.nvm/versions/node/v14.2.0/bin/npm
  Browsers:
  Chrome: 88.0.4324.182
  Edge: Not Found
  Firefox: Not Found
  Safari: 13.1.3
  npmPackages:
  @vuepress/core: 1.8.2
  @vuepress/theme-default: 1.8.2
  vuepress: ^1.5.3 => 1.8.2
  npmGlobalPackages:
  vuepress: Not Found