[another-dave]

Well the article uses the example of swapping `is_published` for `published_at` — with a boolean you could have:

* NULL — never published (e.g. draft) * true — live now * false — previously live but explicitly unpublished

Which you miss if just a date. Similarly he talks about `is_signed_in` — NULL/true/false let's you model the case where a user has never signed in (e.g. an admin created your account but you've never used it) but NULL/timestamp missed this