No matter what day it is that the bill becomes law, that was not intended as the effective date to be an eligible owner for grandfathering. What the date of June 30, 2018 does is establish the point in time at which an owner will be asked to establish, as a matter of fact, that they were a lawful owner, with a duly issued registration certificate for that firearm on a given date.
On June 5th, 2018. See this statement in context.