[pluma]

Yes wrong. The first decade was either 9 or 10 years (depending on whether you want to be weird and include 1 BC), every decade after that was 10 years starting at 'X0 and ending at 'X9. This is how the common usage defines the term.

A lot of concepts in daily human lives get fuzzy near the edges. That's not a failing on humans, that's how language works. This isn't scientific or perfectly rational, yes, but neither are humans.

[gsich]

Yes this is common usage. I'll use it that way too. It's still wrong based on the calendar.

It's not an edge case though.