> 1 AD And this makes things really confusing. 1999 is 20th century even if the year starts with "19". 2000 is still 20th century, 21th century starts in 2001.

> 1 AD And this makes things really confusing. 1999 is 20th century even if the year starts with "19". 2000 is still 20th century, 21th century starts in 2001. "BC" dates feel like negative numbers but they are not. Using closed intervals [10 BC, 10 AD] spans 20 years, but [10 AD, 20 AD] spans 21 years.