The discussions are often silly (especially up to the millennium change in 2000), but the problem has a real origin.
It goes back to 523 AD when the monk Dionysius Exiguus ‘calculated’ the Nativity of Jesus and declared it as the year 1, a one-based numbering. The years before were probably ‘lost’ years and of no interest.
The first decade, century or millennium started January 1, 0001 AD and ended on December 31, 0010 AD, 0100 AD or 1000 AD. Hence, our decade should start on January 1, 2021.
However, what should be and what is really are not always identical. Now, it is accepted everywhere, when the significant digit changes (the tens, hundreds or thousands) the new decade etc. begins.
Another problem subsists:
Many centuries after Exiguus, someone decided to number the BC-years too, an adjacent, negative one-based numbering. The year 0 got lost or was omitted; with our numbering the year BC 1 is followed by the year 1 AD, or written otherwise, year -1 is followed by year +1.
With the positive and negative temperatures of +1° and -1° F or C, the difference is calculated (+1)-(-1) = 2°. Our date system does not allow calculating around the BC/AD limit.
Astronomers, to calculate a historic phenomenon like an eclipse or a conjunction, used their own calendar with the year 0000 existing. Later, with the calculators, they left our dates, in the solar system they calculate with Julian Days (JD), the number of days since the start of the scale on January 1, BC 4713 at 1200 UTC.
Today, January 2, 2020, is JD 2,458,851.