NPR's Rachel Martin talks to Sandi Duncan, managing editor of the Farmers' Almanac, about the debate over when a decade ends, and when a new one technically begins. ...Wow, there is some crazy reasoning.
MARTIN: I mean, it feels like a big deal, 2019 to 2020. Why is there such a debate about whether or not this is the end of the decade?
SANDI DUNCAN: You know, it's really interesting. But I hate to tell you it's not.
MARTIN: It's not?
DUNCAN: Actually, no. We ran a story several years ago. In fact, you know, remember the big celebration in 1999. People thought that the new millennial was going to start the next year. But really, a decade begins actually with the year ending in the numeral one. There was never a year zero. So when we started counting time way back when, it goes one through 10. So a decade is 10 years. So in actuality, the next decade won't start until January 1, 2021.
She says that the twenties will not start until 2021 because there was never a year zero!
There certainly was a year 0. It was 2019 years ago. Nobody called it year 0 at the time, just as nobody called the next year year 1 at the time, as the Christian calendar was only adopted a couple of centuries later.
The year 0 is also called 1 BC, which is confusing, and reflects a poor definition, but is not a reason to say that we need to wait another year to start the twenties.
Apparently somebody thought that a good way to standardize the calendar was to say that Jesus was born on Dec. 25, 1 AD. Mathematicians might have said that it makes more sense to use Jan 1, 0 AD, or maybe the 0th day of the 0th month of the 0th year. Then our calendar years would measure the age of Jesus. But that appears to have been not the intent, as they have him being born near the end of year 1.
Of course estimates of Jesus's birthday could be off by 5 or 10 years. The estimate was just a way of fixing the calendar.