It was my older sister’s birthday yesterday, and it had me thinking about how we talk about birthdays. There’s a peculiar inconsistency in the language we use which I vaguely noticed for the first time a couple of years ago, but never really pursued the thought.
We index our birthdays from 0.
Of course, it is correct that after 10 years alive, we say someone is now 10 years old. That makes perfect sense, and is quite correct. However, we would also describe the day as being their tenth birthday. A second of thought shows this is patently untrue. It is in fact their eleventh birthday. The day they were born was their first birthday. When they turned 1 year old, that was their second birthday. And so on.
The only resolution to this conundrum is accepting that we index our birthdays from 0. Of course the implications of this are essentially nothing, although I suppose this is a reasonable example to provide to confused students in introductory computer science classes who aren’t on board with zero-indexing. However, this realization hasn’t improved my own susceptibility to off-by-one errors.
In either case, it was a small tidbit I thought a few people might find interesting. So when your friend turns 21, undercut the whole thing by wishing them a happy 22nd birthday.