Let's get this straight once and for all: December 31, 2019, is the end of the decade. And yet there are people who will contend that the decade doesn't end until December 31, 2020. Their argument is extremely flawed, and its consequences are easily demonstrated.
For example: David Duchovny and Bono were born in 1960. They were children of the fifties, right?
Of course not, the fifties were from 1950-1959, but to be consistent the 2020 crowd would have to put Dave and Bono in with people born in the 50's, which seems absurd.
What's going on here? A similar argument ensued at the end of the last millenium. Here's a quote from Scientific American magazine at the time: “Years in the most popular calendar used today, the Gregorian calendar, are counted from the year A.D.1. There was no year 0. Before A.D.1 came the year B.C.1. Thus, the first century ran for 100 years from A.D.1 until the end of A.D. 100 ….” (my emphasis). Do you see the fallacy? Choosing B.C.1 or A.D.1 as the first year the century is arbitrary. The argument is about the definition of the century, and the author of the quote slipped in a definition which can and should be challenged.
We can all agree that a year is 365 days, right? Well, no, it's more like 365 and a quarter, give or take. That's why we have leap years. A day is 24 hours, no? Not exactly. That's why we add or subtract a second on our most sensitive clocks from time to time. The point here is the words we use to describe phenomena are not the phenomena themselves. Words are conventions we agree on for convenience.
A decade is ten years – any ten years. A century is 100 years – any 100 years - and so on. When the time of Christ's birth was estimated by Dionysius in 525 A.D. the idea of a year zero was not available. So he estimated that Christ was born on December 25th of the year 1 (he was probably in error), and the confusion of cardinal and ordinal numbers began. If you think about it in Dionysius' way, a week later on January 1st Christ was in his second year, meaning he was two. Perhaps that is the miracle of the Virgin Birth.
When I was in elementary school we had BC and AD to designate years on the calendar. We thought they meant Before Christ and After Death, respectively, but later learned AD meant “Anno Domini” which was Latin for Year of Our Lord.
So December 25th of the year 1 was the beginning of the first Year of Our Lord. And, therefore, December 24th of the year 1 was Before Christ, no? So wasn't that year 1 B.C.? You can see the problem here with trying to get all categorical on the subject of decades, centuries and millennia.
Some people would like to appeal to a higher authority such as Scientific American to settle the argument, but when it comes to words, there is either consensus or a lack of consensus. There is no higher authority than what we commonly agree words mean. Even Moscow Mitch's Republican Senate can't make Trump “innocent”. They can only acquit him. He will still be guilty as sin except inside his cult, where they will see him as being as white as the driven snow.
The second decade of the 21st century will end at midnight December 31, 2019. Enjoy it while you can.
