Why is it that a baby born on Dec. 30, 2017 in South Korea turns two years old, just two days later?

Welcome to the crazy system of ‘Korean age.’ This is how it works: A baby is one year old at birth, and gains an additional year on Jan. 1 (not on birthdays). This method is called the ‘counting age’ system (saeneun nayi) in Korean; it originated from China centuries ago and spread to various parts of Asia, although it’s not quite clear why this system arose. Some say that the culture of respecting mothers created the age system of counting a fetus’s time inside the womb, although there’s no clear-cut documentation for this theory.

Legally, South Korea uses the international age system — which counts exactly how many years it has been since you were born — but informally in daily life, everyone uses both the international and Korea’s ‘counting age’ systems. Why has the latter stayed, when China, Japan and even North Korea don’t use this system anymore? (Defectors are often confused by this age system when they come to South Korea.)

Nobody knows.

And the confusion doesn’t end here. South Koreans technically have three systems to count their years. In addition to the international and ‘counting age’ systems, South Koreans sometimes use what’s called a ‘year age’ system (yeon nayi) — counting your age by simply subtracting the current year from the one you were born. So Psy, who was born on Dec. 31, 1977, would be 40 in international age, 41 in ‘year age,’ and 42 under the ‘counting age’ system.

To avoid the confusion, people often ask each other straight up, “Which year were you born?” And many people just use the international age system as they get older.

Cover image: (Source: Will Clayton via Flickr, CC BY 2.0)