In the prolegomena (prefatory remarks) to his published lectures on the Heidelberg Catechism, Zacharias Ursinus (1534–83), the primary author of the catechism and the one authorized by Frederick III to explain the catechism wrote:

“The doctrine of the church consists of two parts: the Law, and the Gospel; in which we have comprehended the sum and substance of the sacred Scriptures. The law is called the Decalogue, and the gospel is the doctrine concerning Christ the mediator, and the free remission of sins, through faith. This division of the doctrine of the church is established by these plain and forcible arguments.”

Please note that, for Ursinus, law here does not refer merely to a historical epoch (episode or period). It refers to a kind of speech. Yes, he points to a particular, historical expression of that sort of speech but he calls attention to it not because of when it was spoken but because of what sort of speech it was.

The same is true of his account of it’s opposite. What is basic to this distinction is not when the doctrine of Christ as mediator, the remission of sins, and justification through faith alone was given but that it was given and that it is distinct from the former.

This is how Ursinus expressed the “divisio doctrinae:”

The principal Differences between these two parts of the doctrine of the church, consist in these three things:

1. In the subject, or general character of the doctrine, peculiar to each. The law prescribes and enjoins what is to be done, and forbids what ought to be avoided: whilst the gospel announces the free remission of sin, through and for the sake of Christ.

2. In the manner of the revelation peculiar to each. The law is known from nature; the gospel is divinely revealed.

3. In the promises which they make to man. The law promises life upon the condition of perfect obedience; the gospel, on the condition of faith in Christ and the commencement of new obedience. Hereafter, however, more will be said upon this subject in the proper place.

Please notice that what distinguishes the law and the gospel is not when they were articulated but what they are or what they teach and require. The distinction is theological and hermeneutical.

These distinctions are basic to Reformed theology. They cannot be marginalized as an idiosyncratic view of one or two early Reformed writers who were overly influenced by Lutheranism. This distinction was made consistently in the classical period and by orthodox writers in the 18th, 19th, and 20th centuries. Here’s a resource page for more.