I think historically it will be remembered when AI starts to make impact for general populace. Random street people has not even heard about these advances which we have today. History always remembers big happenings so I would say the moment general populace start to really losing jobs will be the year which will be remembered. It might be some big catastrophe/event related to AI which marks the moment. Something which causes shock to whole world. Something like collapse of soviet union, 11/9, 2008 financial crisis etc. It has to be something big.