No, paulz; dates do not have zero (as sft42 said, the modern reckoning of historical dates is that 1BC was followed immediately by 1AD, and there was no year 0) but ages do. Similarly, the millenium started on 1st January 2001 - not on the 0th of January (January being the 1st month of the year, not the 0th). When a baby is born, it is zero years old but it is already in the first year of its life. Just as it takes the baby a whole year to reach its first birthday, it takes a century a whole hundred years before it finishes. So the first century started with 1AD and took 100 years (not 99 years) to finish with 100AD.