The date called the beginning usually belongs to whoever wrote the calendar.

Beginnings are chosen. Whoever writes the calendar decides which event starts the story, and everything before it becomes background, prelude, or nothing. The choice looks like a fact once it has been in use for a while, because dates feel arithmetic rather than editorial. A different starting point would make a different account of the same events.

When you are told when something began, ask what was happening just before. In your own life the same trick works, because the moment you date a change from is a choice that shapes the story you tell about it. Move the starting line and the meaning often moves with it.