What technique is this? I am unaware of any Office of Decades charged with deciding precisely what constitutes the sole definition of "decade".

> I am unaware of any Office of Decades charged with deciding precisely what constitutes the sole definition of "decade"

ISO 8601 defines what a 'decade' is.

I mean you're free to ignore it of course, like you're free to say March has 34 days if you want... but you may have trouble communicating with people and integrating with existing systems.

8601's definition of a decade is somewhat different from common parlance. They're using it only to mean "the first digit of a two-digit year", and doesn't really apply it beyond that. If you stick to only that definition of a decade, you're going to have communication problems with human beings.

Nah. 99/100 people will say the 90's decade is 1990-1999

A decade is just 10 years. There’s no specific start or end.

https://en.m.wikipedia.org/wiki/Decade

> A decade is a period of 10 years.

That's for informal use in speech, not what anyone building a formal system (the context of this thread) means.