Some people argued that 2000 was not the start of the new millennium. Their argument was that the new millennium began in 2001. I think these same people are now arguing that we are not in the new decade yet, not until 2011.
I think my idea of an explanation works.
If there is no zero year, if it's not counted as a number and 2001 was the first year of the new millennium and 2011 is the first year of the new millennium can I ask, how old is a baby when it is 1 hour or 364 days old? The baby is not one year old. The baby is zero years old. However, the baby is in its first year.
When the child is 10 the child has lived ten years. The second after the child is 10 it is in its eleventh year, it's second decade of life.
Does that work as an explanation? Tell me if I've missed something.
Oh crumbs. Now I've looked up BC and AD and I see that there was no year zero. It jumped from 1BC to 1AD... oh noes!