Call it what you want, but for the next TEN years, the date will end with 201_, beginning yesterday, and THAT’s a fact.
I’m on your side! The 0 decade started with 00 and goes through then end of 09. Decades means 10 years, and 00 through 09 is 10 years. To me it has to do with the way the years are numbered. In our counting system when you bump nine by one you replace it with a zero. Seems open and shut to me, but I can see how some would see it the other way too!
It’s like the old trick you play on a kid to prove that he you have 11 fingers. Hold up your left hand, fingers extended. Start with the thumb on that hand and number the fingers 10, 9, 8, 7, 6. Then hold up the 5 fingers of the other other hand “Plus 5 equals 11!” One of the neatest tricks in the book for kids and adults.
It has to do with the difference in counting and numbering. They are not the same!