Many books and works of art printed over the centuries by hand-operated presses are undated or poorly dated. The use of classical methods of dating, such as matching watermarks in paper, has had limited success. Here, evidence is presented that the woodblocks and metal plates used in printing deteriorated at a constant rate, useful for time estimation. This change was measured in 2674 Renaissance prints. In successive editions, woodblock prints increased in the number of line breaks, whereas copperplate prints became paler. In both cases, the change was time dependent, not print dependent. For woodblocks, this can be explained by the aging of the wood, creating breaks in lines. For copperplates, image fading is a result of line thinning caused by surface erosion of the plate, not from compression by the printing press. The time dependency in copperplates is best explained by corrosion during plate storage, which was removed before each printing by polishing (erosion). The average rate of copperplate deterioration, 1–2 μm yr−1, was estimated from the thinning rates of lines in two Italian prints and agrees with typical rates of atmospheric corrosion of copper. The print clock, if it applies generally, offers a relatively simple means of dating early books and prints.