Of course, you need to import FLOOR_DATE. For use in a template, you may need to use security declarations to make this possible. For those cases, I have a module public with imports that should be available to restricted contexts and make its import possible with an allow_module("public").
Why don’t you use EffectiveDate()? This one returns either None or the effective_date set on the content. With effective() you get either the content’s effective_date or creation_date or FLOOR_DATE.