Re: 1970s?
Since the AD chronology was devised in 525, we either have to accept that decades, centuries and millenia begin and end at 5, 25 or 525 years past the obvious numerical boundary (*), or we have to take a deep breath and conclude that the choice of epoch is completely retrospective and, being numerate, we may as well pick the sensible one. So ... There was a year zero, just like there was a year minus one. People at the time didn't call it that, but they didn't call year one by that name either, or even year five hundred and twenty four.
(* Actually, I'm rather tempted by this. Its principal merit is that it would really annoy certain people.)