Today is the start of the new millennium.
Some, like Dick Teresi, have argued that this is the result of a number of errors in year counting committed in the past, in the middle ages in fact. There never was a year 0, we went from 1 B.C. to 1 A.D. He says there should have been a year 0 however between those two years, just like there is a year 2000 between 1999 and 2001.
However, others argue that because years B.C. are counted starting from -1, there is no room for a year zero, just like on a Carthesian coordinate system, where zero is a point, not a time interval.
Be that as it may, we are stuck with that, unless we decide to renumber all years from 1 B.C. backward, so that 1 B.C. becomes the year 0, 2 B.C. become 1 B.C. and so on.
Since we are likely to stay with the current counting system for a while... pop the Champagne today!