In his Commentary on the 26th Question of the Heidelberg Catechism, Ursinus gives a brief explanation of differing views among the Reformers concerning the age of the Earth. He claims a “common reckoning” of Biblical chronology which allows a conclusion that the world was 5,534 years old. He writes:

Lastly, God created the world, not eternally, but at a certain and definite time; and, therefore, in the beginning of time. “In the beginning God created the heavens and earth.” (Gen. 1:1.) According to the common reckoning, it is now, counting from this 1616 of Christ, 5,534 years since the creation of the world. (pg. 145, P&R reproduction of the 2nd American edition)

Of course, there was some discrepancy among the calculations. Ursinus goes on to list the difference of opinions between Melanchthon, Luther, the teachers at the school of Geneva, and Matthieu Brouard. Melanchthon claimed that the Earth was 5,579 years old, Luther argued that it was 5,576, The Geneva teachers claimed 5,559, and Beroaldus said 5,545. But Urinsus is not overly troubled:

These calculations harmonize sufficiently with each other in the larger numbers, although some years are either added or wanting in the smaller numbers. According to these four calculations, made by the most learned men of our times, it will appear, by comparing them together, that the world was created by God at least not much over 5,559 or 5,579 years. The world, therefore, was not created from everlasting, but had a beginning.

Now, the fact that Ursinsus and the other Reformers held these views is no proof of Young Earth Creationism. It does not even mean that Young Earth Creationism is required by the Reformed Confessions. But what it does mean is that Young Earth Creationism is not a modern or novel element within the mainstream of Protestantism. The men cited are, as Ursinus writes, “the most learned men” of the times. They are making their calculations not based primarily on natural science but rather exegesis. And while the development of modern science, particularly physics and geology, is important, we should not then jump to the conclusion that reading the historical details of the Bible, and especially the chronologies of Genesis, in a literal fashion is some sort of obviously naive and ridiculous thing to do. It was, in fact, the default position and the prima facie interpretation of the text during the time of the Reformation.