warning from CI when adding a new blog post with wrong date

[nikomatsakis]

Since blog post filenames must include the date, and posts often stay in draft form for days, it's easy to merge with the file having the wrong name. It'd be so great if we could detect and prevent that somehow.

[Mark-Simulacrum]

One question I have had historically is whether we can just drop that requirement and instead have some intelligent "when was the file added to git" support or something like that. We ~never merge things ahead of time or into the past, anyway...