…did not exist.
At least, neither in the Julian or Gregorian calendars – the reason being, the Romans had no concept of zero (they did not play football).
Which means decades, centuries and millennia last until the end of the “0” year .
Thus the Sixties ACTUALLY lasted from the 1st of January, 1961 – until the 31st of December, 1970.
And all those chumps who celebrated The Millennium on the 31st of December, 1999 – were a year EARLY.
But then again, these were just NUMBERS. The REAL Sixties did not get underway until 1963, when pirate radio and the Beatles arrived – and they continued until the opening months of 1973.
The Sixties were a CONCEPT, man.
Likewise, the Seventies lasted until 1982. The Eighties until 1989. And the bloody Nineties are STILL WITH US.
As for The (popular) Millennium – despite fears of the World ending, courtesy of a Russian nuclear first strike being triggered by its ageing computers, the only thing that happened (apart from a few ATMs conking out) was all the little NUMBERS turned over.
It was like the odometer of a car. When it passes the 999999 point, the bonnet does not suddenly fly up and the doors fall off – unless you are driving a clown car.
Or you were so intent on WATCHING the odometer turn over – you rear-ended the car in FRONT.