By mistaking the count of "one" as marking the beginning of a unit when it clearly marks the end. And by asserting that because zero has no content it also has no function.
This is not, as Dr Williams maintains, the 1,999th year since the birth of Christ but the 2,000th, hence our description of the current century as the 20th century. And the reason none of us claim to living in our "0th year" is that the time from our birth until our first birthday is our 1st year.
If Dr Williams's view were correct, the first century would have begun on 1 January in the year 0001 and his own life would have begun when he reached the age of one - clearly preposterous suggestions.
The first century began on 1 January in the year 000, and Dr Williams's first year began when he was born and ended when he reached the age of one.
Similarly, the 21st century will begin on 1 January in the year 2000, and on 1 January 2001, both the 21 century and the third millennium will be one year old, so can we please have an end to these illogical and rather pointless assertions.