You also won’t win the debate by simply sticking your fingers in your ears and repeating this mantra, ignoring the numerous objections to your claim that there is a cut-and-dried “right” and “wrong” way. You’ll just get earwax on your fingertips.
How about going with the common definition that most normal people use then? Wikipedia certainly has no ambiguity about the definitions of decade, century, or millennium, even if you do.
https://uploads.tapatalk-cdn.com/20180107/8c530e1dd02e6e99e79041b2c06175c9.jpg
https://uploads.tapatalk-cdn.com/20180107/d63254a61c41ecef37c5bcf5503cb746.jpg
Absolutely fine with me. Most normal people celebrated the start of the new millennium on Jan 1st 2000, so the common definition of the start of the new millennium is Jan 1st 2000. I guess we can put this to bed then.
In happier news: The 21st was a leap century; most centuries have only 36524 days, but the 21st century had an extra day!
Spend it wisely.
BBC radio has just announced that there will be an interview soon of someone born in the 21st century who is now an adult.
So BBC and Sir Winston Churchill (and, per OP, President Modi of the former Raj) are all on the side of common sense.
Maybe it’s another tea vs coffee thing.