Just two hours after Lee Dong Kil’s daughter was born on New Year’s Eve, the clock struck midnight, 2019 was ushered in, and the infant became 2-years-old. She wasn’t alone, though it happened for her quicker than most: every baby born in South Korea last year became 2 on January 1.

According to one of the world’s most unusual age-calculating systems, South Korean babies become 1 on the day of their birth and then get an additional year tacked on when the calendar hits January 1. A lawmaker is working now to overturn the centuries-old tradition amid complaints that it’s an anachronistic, time-wasting custom.

Ancient calender

For parents whose babies are born in December, it can be especially painful. One hour after his daughter’s birth in the central city of Daejeon at 10 p.m. on Dec. 31 last year, Mr. Lee posted the news on social media. His friends immediately showered him with congratulatory messages.

“An hour later, when the New Year began, they phoned me again to say congratulations for my baby becoming 2-years-old,” said Mr. Lee, who is 32 internationally but 34 in South Korea. “I thought, ‘Ah, right. She’s now 2 years old, though it’s been only two hours since she was born. What the heck!’”

The origins of this age reckoning system aren’t clear. Being 1 upon birth may be linked to the time babies spend in their mothers’ wombs or to an ancient Asian numerical system that didn’t have the concept of zero.

Becoming a year older on January 1? That’s even harder to explain.

It could be that ancient Koreans cared a lot about the year in which they were born in the Chinese 60-year cycle, but, without regular calendars, didn’t care much about the specific day they were born; so they mostly ignored the day of their birth and instead marked another year of age on the day of the Lunar New Year, according to senior curator Jung Yonhak at the National Folk Museum of Korea.

North Korean twist

This may have then shifted to the solar New Year on January 1 as the South began embracing the Western calendar. North Korea uses the Western age calculating system, but they have a twist: they follow their own calendar that’s based on the birth of national founder and president-for-life Kim Il Sung.

The year of your birth is still incredibly important in South Korea, and lumps those linked children together for life.

Other Asian countries, including Japan and Vietnam, abandoned the Chinese-style age system amid an influx of Western culture. Officially, South Korea has used Western-style calculations since the early 1960s. But its citizens still embrace the old-fashioned system.

Some South Koreans still worry that the practice makes their nation look odd on the international stage.

Ahn Chang-gun, from the southeastern city of Gimhae, said he felt “empty” when his first child became 2 on January 1, 2013, about two weeks after his wife delivered him after eight years of marriage. “He was this precious baby that we finally had, but I felt that all of a sudden two years had just gone by and yet I hadn’t done anything for my baby,” said Ms. Ahn.

In January, lawmaker Hwang Ju-hong tabled a bill aimed at requiring the government to put international ages in official documents and encouraging general citizens to go with their international ages in everyday life.