It is the standard way of writing the century in many European countries/languages.
20th century in roman numerals
I think you're thinking of MM, which was the last year of the 20th century.
We're still a few years out from the 2000th century.
That would be MCMXCIX
No, 2000 was the last year. There was no year zero.
Hmm, interesting - I stand corrected. So the first millennium was years 1 through 1000. First I've heard of it!
Another casualty of 1-based indexing I suppose. Perhaps the people setting the system up didn't have a concept of 'zero'?
There is a Seinfeld episode that covers this topic.