South Korea is a modern society. The nation is a leader in electronics and robotics and an e-sports powerhouse. But start talking about birthdays, and you might wonder whether it’s trapped in the Dark Ages. Now some lawmakers hope to overturn an ancient tradition for figuring age.

For centuries, Koreans have used one of the world’s strangest calculating systems to determine age. Some people call it “lunar age” or “East Asian age.” To Koreans it’s simply “Korean age.” In the past, other Asian countries used it too. Today, South Korea is the only country using this method.

According to Korean age, a baby is one year old at birth. The baby’s age increases a year on New Year’s Day—no matter when its birthday is. It’s similar to how Jesus’ time in the tomb is counted. The Bible says He spent three days there. Even though He was buried late on Friday, the whole Friday counts as a day. He rose early on Sunday morning, but the whole Sunday is counted. Similarly, Korea counts the entire year in which a person lives even a day.

Still confused? Here’s an example: Lee Dong Kil’s daughter was born at 10 p.m. last New Year’s Eve. At that moment, she was considered one. After midnight—the new year—she got to count another year in her age. Lee’s friends congratulated him on having a two-year-old.

“I thought, ‘Ah, right. She’s now two years old, though it’s been only two hours since she was born,’” says Lee. He is 32 internationally but 34 in Korean age.

In January, lawmaker Hwang Ju-hong sponsored a bill to end Korean age. His bill would require the government to put international ages on official documents and encourage citizens to switch too.

Officially, South Korea has used Western-style age calculations since the early 1960s. But most citizens still embrace the old-fashioned system. They’re used to living with two ages.

The origins of the system aren’t clear. Some scholars say ancient Koreans cared a lot about the year they were born—but not about the specific day. Others think the tradition relates to an early number system without the concept of zero. Yet another view links it to the time babies spend in their mothers’ wombs—a time Psalm 139:13 refers to as being “knitted together.”

Whatever the history, some South Koreans say that Korean age is problematic. It groups children born as much as 364 days apart into the same classes—even if the younger ones aren’t ready.