... For any feature that you might take as core to the identity of "the Roman empire" you'll find different times for when they began and ceased to apply...
Somewhat OT, but I find this sort of thing fascinating, as our popular modern perception of a given era in the past is usually an eclectic amalgamation of different customs, governments, people, etc. from time periods that were too far apart to have interacted with each other, haha.
Using a modern example with a similar dynamic but that uses geography rather than time periods, it is kind of like how your average American's stereotype of the "archetypal German" is this hilariously contradictory person who is simultaneously a jolly Bavarian in his Lederhosen while also being an obnoxiously prompt Prussian obsessed with order and who is "no fun."