Completely spitballing, it’s possible the Republic could have transitioned into the Empire gradually enough that you really can’t strictly say Year X was the Republic, Year Y was the Empire. There’s nothing textual that supports this, but it fits in with the metaphor for American Imperialism.

yotsuya said:
This firmly plants the start of the Empire at their birth. Why? Vader did not know Luke had a sister which means he was not present at the birth. So the event that turned Vader and started the Empire happened between conception and birth of the twins.

I don’t think we can necessarily assume that Vader turning to the dark side and the rise of the Empire happened at the same time.