The Korean age calculation system differs from the chronological system used by the majority of countries in the globe. The Korean age is 1 or 2 years older than the chronological age.

Under the chronological approach, your age (the "actual age") indicates the actual amount of time you have lived since birth. For example, if you were born on December 31, 2014, and it is now January 12, 2015, you are only 12 days old, and you will be a year old on December 31, 2015.

Under the Korean system, you are already a year old when you are born because the Koreans consider the time you spent in your mother's womb (albeit that length of time is just 10 months). Then, instead of your birthday, you will be one year older on January 1st each year. As a result, while you may throw a birthday party to commemorate your birthday, the Korean system does not include your birthday for computing your age. For example, if you were born on December 31, 2014, you were already one year old on that date. You acquired one year on January 1, 2015, and are now two years old (January 12, 2015). Your age will not change on your birthday, which is December 31, 2015, so you will still be two years old.
As a result, depending on the day you compute your age, your Korean age is either one or two years older than your real age. If you calculate your age before your birthday, it will be two years older than your actual age; if you calculate it on or after your birthday, it will be one year older.

For legal purposes, Koreans employ the actual age rather than the Korean age. To avoid confusion, they may include the word 만 (man), which means "full", before the age to make it apparent that they are using the actual age.

To avoid confusion, when discussing age with Koreans in South Korea, I used the term 만 before the age to signify the true age or told them the year I was born.