Changing a boolean database field like 'is_confirmed' to a nullable datetime is a simple, cheap hack that records a little bit of information about an event. It's appropriate when you're not sure you care about the event.

If you know you actually care about the event, there are probably more fields to stuff into an event record, and then maybe you could save the event record's id instead?

But going too far in this direction based on speculation about what information you might need later is going to complicate the schema.

See Paul's comment in the other thread for more: https://news.ycombinator.com/item?id=44423995