Date and Time Systems

[Kennebec_Captain]

Here’s a good answer:

@Urs

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.