The string -> Date conversion happens automatically for all types, right? So no need to do anything?

I believe it works automatically on keys suffixed with At, so createdAt should work fine. Best way to be sure is to write a test case (or at least try it out locally with the changes).

Do we use explicit T | null anywhere if the API returns the type or null? Seems we like to do T? rather

I personally prefer using optional fields (so undefined) for better ergonomics, but if the API returns null somewhere, we need to have it in the type, and if a user can pass null somewhere explicitly, we need to allow it in that type too.

I tested the Date conversion locally and double-checked the explicit nulls