OMG, great thread!
For me, the correct answer is that the "new millenium" starts with the first day of the year 2000.
This is simple. And simple solutions, methods, and concepts are good ones.
So, IMO,
we are in the 22nd year of the new millenial... on our way to an age counter of 2022.
When you are born, you are not age 1 (in NA), you are age 0.
At the END of your first year, you turn "age 1".
This (important!) is the basis for people's understanding of years.
After your first year, you turn age 1.
So... importantly,
the masses decided to celebrate the new millenial on Jan 1, 2000.
After the first year of the millenium, we go to 2001.
And you know what... I'm fine with this... and everybody should be.
Now - the rational for years based on BC/AD is also correct.
But this is also
way back in the past when mankind was doing a bunch of stuff that isn't socially or even factually accepted today.
Heck - pretty sure EVERYBODY in the year 0 thought the world was flat.
So... eff that. And it's also a bit religion-based... which we know (sorry to some) doesn't align with scientific research.
It's a bit the same as the TOMATO debate.
Scientifically - a tomato is a fruit (seeds).
Nutritionally and by supreme court - a tomato is classified as a vegetable.
Socially - kids are raised with tomatoes being more a part of a "salad" than as something you eat as a "healthy snack" in the same way as a banana, apple, pear, or orange.
So... by the masses, I think most view tomato as a "vegetable".
The effin masses rule our definitions these days.
So if the effin masses turn age 1 after their first living year...
Then the effin masses shall say that we are currently in the 22nd year of the new millenial, because at the end we'll turn to 2022.