Why is it called your eighteenth birthday when you turn eighteen Wouldn't it be your nineteenth actual birthday because the date appeared the nineteenth time including the day you were born?

Answer 1

That is true but what people mean is it has been eighteen years since your birthday.

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The point of birthdays is to observe how old someone is. People are considered to be zero years old when they are born (although if you wanted to, you could date their lives from conception instead, which makes them usually 9 months old when they are born - but that is not the normal convention for calculating age). You have to live another year to be one year old and celebrate your first birthday - even though if you count the day you were born as your first birthday, a year later would actually be your second birthday. But you are still just one year old.