Elegran: "So what would you call the century between the year 0 and the year 100 AD, the one that was the first hundred years (ie, century) after Christ? Would you call it the first century or the nothingness century?"
In our dating system they call that the 1st century. It's like your age. When you are born you are in your first year, after your first birthday you are in your second year, and so on. That's how the years named 19xx were called the 20th century, and the ones we're in now, named 20xx, are called the 21st century.
One point, though. In the system we use there isn't a year called 'year 0'. There's the year 1 BC, and the next one is the year 1 AD. The first year of the 1st century was 1 AD, and the last year of the 1st century was 100 AD. The first year of the 2nd century was 101 AD, and so on.
The consequence of this is that the first year of the 20th century was 1901 AD, the last year of the 20th century was 2000 AD, and the first year of the 21st century was 2001 AD. It also means that the first year of the first millennium was 1 AD, the first year of the second millennium was 1001 AD, and the first year of the third millennium was 2001 AD.
So all those people who celebrated the millennium changeover at 1999/2000 were a year too early.