For instance my birthdate is january 20th 1993 so do I add 1 year or 2?
Answer on So i'm going to korea for a family trip and i noticed they have an age system so how exactly does it work?
One's age changes on the calendar year, meaning Jan. 1, not your birthday. Think of it as "calendar years of birth" when determining age. For example, if someone was born December 1, 2001, they would be age "1" for only a month, then turn "2" on January 1, 2002, "3" on Jan. 1 '03, "4" on Jan. 1, '04 and so on.
For those using western ages, it is pretty simple. Take your western age, and go up by one year, but on Jan. 1 before your birthday date. Since you are Jan. 20 you are only going to have to change your age 20 days in advance of what is usual. For people born later in the year, it can seem confusing at first, so for example, if someone will turn 18 in 2011, but have not yet had there birthday yet, they are still 19 for the entire year, even though for part of the year they are 17 in the west and 18 after there birthday in the west.
However, if you look western, either by race or style, Koreans normally assume you mean the western age when they ask you. So basically, when answering in English your age is still "eighteen" to Koreans, however, if you want to be fun, you can answer in Korean, which then you say "???" or "Yeol-ahop" which means "19" (which you would have also said from Jan. 1-19 this year as well.)
Also, note, for public records, Koreans actually list their age by the western standard (calculating age by years from your birthdate) that is used in the U.S. and just about everywhere else in the world; the old way is done more for traditional purposes in conversation, not as legal matters.
Note, there has been a big trend in Asian countries to do away with this method of calculating age (also note that originally the lunar new year was the marker not the western year), but except for Korea and some parts of rural China, it is all but gone for except with the oldest generations. Part of this is because, while originally being thought older meant wisdom, most people prefer to be thought younger (like who wants to turn 40 earlier when you can wait more than a year!)