It is a fairly common practice, particularly in relation to the Firearms Act. The firearms grandfathering provisions are all related to a point in time that the person possessed that firearm.
It also occurs in other types of legislation, most notably in tax legislation. The eligible day on which a person must own or not own certain shares or other types of investments is a day that is listed in the legislation, for the sake of transparency and to ensure that there are equal public signals sent to everyone who is engaged in that marketplace. That day is presented as a date that is not linked to coming into force, and does not presuppose coming into force or royal assent by that day. In fact, in the case of tax legislation it is often budget day, the day on which the government makes its intent clear.
The analogy in Bill C-71 is that the government has made a very clear intent about what would be the commencement day or the eligibility day, irrespective of when or if the bill receives royal assent and is brought into force.