It’s more a formality I think. Since recording of atomic time started in the early 1970s it’s accepted that an atomic second is an actual second back as far as

It’s more a formality I think. Since recording of atomic time started in the early 1970s it’s accepted that an atomic second is an actual second back as far as 1970. No assertion of accuracy is made beyond that. Of course otherwise you can negative offset as much as you like beyond that but because of physics and stuff it can’t be treated with the same level of certainty or something.