Good point. However, approaching this problem from “YAGNI” point of view is a bit misleading, I think. If you are not going to need the timestamp, you shouldn’t add it to your code base.

In my opinion, hastiness is the culprit. When a property appears to be a binary one, we jump to the conclusion to use a boolean way too quickly. We should instead stop and ask ourselves if we are really dealing with a situation that can be reduced to a single bit. The point raised by the article is a good example: you may want to record the state change as timestamp. Moreover, in a lot of the cases, the answer is not even binary. The values for is_published may be, “Yes”, “No” or “I don’t know” (and then we will be too quick to assign null to “I don’t know”). Underlying problem is that we don’t spend enough time when modeling our problems. And this is a sure way of accumulating technical debt.

Ehhh, I don't quite agree with this. I've done the same thing where I used a timestamp field to replace a boolean. However, they are technically not the same thing. In databases, boolean fields can be nullable so you actually have 3-valued boolean logic: true, false, and null. You can technically only replace a non-nullable field to a timestamp column because you are treating null in timestamp as false.

Two examples:

1. A table of generated documents for employees to sign. There's a field where they need to agree to something, but it's optional. You want to differentiate between employees who agreed, employees who disagreed, and employees who have yet to agree. You can't change the column from is_agreed to agreed_at.

2. Adding a boolean column to an existing table. These columns need to either default to an value (which is fair) or be nullable.