Imagine a child born on January 1 in the year 4 BC. In 3 BC he turns one year old. In 2 BC he turns two years old. In 1 BC he turns three years old. In 1 AD he turns four years old. In 2 AD he turns five years old. On January 1 in 2 AD, how many years has it been since he was born? Five years, obviously. But this isn’t what you get if you subtract the years: 2 – (– 4)=6 years old. You get the wrong answer because there is no year zero.
