When Lee Jae-hye goes to the United States, she is 30. When she’s back in South Korea, she is 32.

"It’s so confusing,” said Lee, a video producer in Seoul who frequently flies between the two countries.

That is because South Korea counts people’s ages three ways, often adding a year or two to the international standard. This can present situations ripe for confusion, since age determines roles in the social hierarchy and is important in legal milestones such as when one has the right to drink or vote. It undergirds mundane tasks like filling out official paperwork, and it is key to figuring out how to address elders.