The year zero does not exist in the Anno Domini (AD) system commonly used to number years in the Gregorian calendar [1], so the first decade was years 1AD-10AD (ten years). This means current decade started on January 1st 2001. and ends with December 31st 2020.

From Wikipedia Decade[2] article:

There are two main methods of counting decades in recognition. One, called ordinal, counts decades starting with the first year 1 CE (For example, the years 1981–1990 is referred to as the 199th decade or the 9th decade of the 20th century), while the other, called cardinal, groups years based on having the same digits (For example, the years 1980–1989 is referred to as the 1980s, or commonly known as the eighties).

1: https://en.wikipedia.org/wiki/Year_zero

2: https://en.wikipedia.org/wiki/Decade

According to what years mean. Years are not 0 indexed.

A "decade" of apples would be apple #1 to apple #10 The second "decade" would then be apple #11 to apple #20

Ergo, 2020 is the ending year of the second decade of the third millennium, which started on 2001 :)

It's essentially the difference between celebrating a new chunk of time defined by our collective radix, and celebrating a year because of the way it looks.

We are now in "the 2020s" but not the second decade until next year. Therefore, it is inaccurate to say we are in a new decade.

They are zero indexed. The first zero is just missing. It's only relevant if you need to work in terms of the nth century, or cross the boundary between CE and BCE.

Calling this the start of a new decade, and 2001 the start of this century, still makes perfect sense with the logical and consistent model that ISO has come up with.

https://www.iso.org/obp/ui#iso:std:iso:8601:-1:ed-1:v1:en

I like ordinals and appreciate the distinction between ordinal and cardinal, and am happy with this edge case.

It gets confusing with centuries and terms like the 2000s (https://en.wikipedia.org/wiki/2000s) but decades tend to always be called 10s, 20s, etc. If you can get people to call this the 202nd decade, more power to you. However, defining the 2020s or 20s as 2021-2030 is wrong.

> it doesn't make any sense. According to who?

The year numbering in the current western calendar starts from one (AD 1), not zero (it jumps from 1 BC to AD 1). So the first decade would be from AD 1 to AD 10, the second decade from AD 11 to AD 20, and so on (and the same on the other direction, with a decade from 10 BC to 1 BC, another one from 20 BC to 11 BC, etc).

Re: billions of people

being in majority ≠ being right

Language is defined by the majority.

This isn't an issue of language, it's a check on the internal consistency of a number system. No year zero and the decade starts next year are internally consistent. That most people think the decade started doesn't make it true, just like most people believing in God(tm) doesn't make it true.

According to decimal numbers. 1-10, 11-20, 21-30, etc.

You could argue 0-9, 10-19, 11-20 as an equally vaild grouping.

Not if you're counting years, since years are not 0 indexed