Why a Korean baby born on Dec. 31 turns two years old the next day

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When you arrive in Korea, you age a year, or two.

For hundreds of years, Koreans have used a different method to count age than most of the world. It’s sometimes referred to as lunar age, nominal age, or East Asian age reckoning, but in Korea, it’s just called Korean age. The method has its origins in China before spreading to different parts of Asia, but today, South Korea is the only country where everyone still uses it.

A person’s Korean age goes up a year on new year’s day, not on his or her birthday. So when a baby is born on Dec. 31, he or she actually turns two the very next day, as illustrated in the video above. If you want to find out what yours is, follow this simple equation: Take the current year, subtract your birth year, and add one.