For me, the start of a new decade is the first moments of a year that ends in 1. But there are 365+ days before that new decade registers up the '1' digit. So it's the time space between the 0 and 1 that is important. The 0 is the start off point. The 1 is the completion. And so on.
With this logic, the 1990 World Cup was in the 80s then?


