Think of as "After Birth of the Lord".
Anno Domini, "Year of the Lord['s birth]", was shorter. And avoided a messy afterbirth reference.
The fifties flowed into the "sixties" flowed into the seventies flowed into the eighties.
There is no precise point in time that cultural eras shifted from one to the next.
I, myself, do not associate Rubik's cubes with the seventies. Definitely eighties.
Arguing about zero or one being the first of a sequence. Was this ever an issue before Computers?