Rosenzweig (Judaism Despite Christianity: The 1916 Wartime Correspondence Between Eugen Rosenstock-Huessy and Franz Rosenzweig, 157) offers this profound rejoinder to the professorial habit of trying to modify the traditional epochs of history:
“It is necessary . . . to accept the traditional periods, and to avoid wanting to be ‘original’ like the professors - and like ourselves when we were still professorial. 1789, 1453, (1517), 476, (313) - these are truths, nay the essence of history, just because they are traditions. To present one’s own epoch is only possible with permanent reference to this system of coordinates of traditional historical truth.”
It’s the after-story that makes these dates critical. Whatever “actually happened” on December 31, 1517, that date inaugurates a new epoch because it was regarded as starting a new epoch. It initiated what Rosenstock calls a “body of time,” and we know that it did because people perceived that it did.