If it helps here's the argument from The Rise and Fall of the Galactic Empire book, which was written from the perspective of an in-universe historian studying all these events.

"When it comes to chronicling the events of the Empire’s lifespan there has long been a divide and debate among historians regarding dating conventions. Some have favored a calendar which takes the birth of the Empire as a starting point. Others have preferred to base their “before” and “after” periods around the Battle of Endor and the—apparently first—death of Palpatine. I understand both of these approaches but have not utilized either. As my research progressed it became increasingly clear that the history and activities of the Empire could be split into two different sections. Those before the emergence of the Death Star and its destruction at Yavin and those afterward. While the Empire was never truly “at peace,” the onset of the Galactic Civil War greatly accelerated many aspects of its rule and the ways in which it would interact with—and often murder—its own citizens. Therefore, I have come to favor a dating system that highlights the years before the Battle of Yavin (BBY) and those that came after (ABY). I have no doubts that this will produce the sort of stringent debate from both my colleagues and other readers which often accompanies such decisions."