21

o.O
How does that work?

To think that you can do this only 2 more years...

That's because the 1st century runs from 1 AD through 100 AD, so subsequent centuries always had to start with 1. And that's because year zero was never a thing in 'our' calendar system.

The year zero does not exist in the Anno Domini (AD) system commonly used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year 1 BC is followed by AD 1.

(...) same as new year starts at 1st Jan 00:00 not 00:01 nor 2nd Jan 00:00

That 0-th hour is a modern mistake, like the 1st year is year 1, not 0, the first hour of the first day of that first year... was the 1st hour, not the 0-th one. That's because Roman people were not knowing the zero at all. They were just unable to say “the 0-th hour”.
So, the very first day started on a first hour (and not a zero one), the same applies for all subsequent days since... as a consequence, new year only begins on its very first second, that's 1st Jan 00:00:01, and not 1st Jan 00:00:00 as you're assuming, because it's midnight of the previous year !

TL;DR: 1st Jan 00:00, besides its misleading numbering, is truly in fact midnight of the previous day (of the previous year).