Celtic star Oh Hyeon-gyu officially become a year younger overnight, due to bizarre age rulings in South Korea.

The country’s previous age-counting method made people a year or two older than they really are – with Wednesday’s change formalising the international method as standard in administrative and civil laws.

Traditionally, South Koreans turn one on the day they are born – and another year older when the calendar hits January 1.

That means someone born on December 31 turns two the next day. But now, following the widely recognised international method, people will turn one a year after their birth.

That means Celtic striker Oh, who was born on April 12 2001, will now be a year younger in his homeland.

Government officials have admitted the new law will not meaningfully change how public services are delivered – with most already based on international ages.

President Yoon Suk Yeol has described standardising ages as a key goal of his government, citing a need to reduce “social and administrative confusion” and disputes.

International ages were already the standard in most South Korean laws and official and legal documents and define when a person goes to school, becomes eligible to drive and vote and gets a pension.

While the new law says a person’s age must be counted by the passing of birthdays for most public services, it does not affect other age-related regulations that are based on yearly rules.

Staying the same is the country’s legal age for drinking and smoking, which are allowed from January 1 of the year a person turns 19 in their international age, regardless of whether their birthday has passed.

The new law does not affect when South Korean men become eligible to serve their mandatory military duties, which is from January 1 of the year they turn 18 in international age.

Changing those age regulations would require revisions of the country’s youth protection and military service laws, the government legislation ministry said.

Lee Wan-kyu, the government legislation minister, said in a statement the new law is mostly aimed at reducing confusion in daily life and inspiring a change in “social perception” toward a more rational way of counting ages.

Promoting international age as a social standard could be important in areas like health care.

For example, a child could be at risk if his or her parents see a cough syrup instruction that reads “20ml for 12 years and older” and think it means the so-called “Korean age”, the ministry said in a press release.

There have also been instances where public transport users demand refunds after paying for their children’s fares, thinking the free rides given to children under six meant their Korean age.

Differing age interpretations inspired a major dispute in 2004 at a dairy company, Namyang, after unionists and management disagreed over the terms of their collective bargaining agreement that allowed the company to gradually reduce the salaries of employees aged 56 or older.

Following a lengthy court battle over whether 56 meant the Korean age or international age, the Supreme Court in 2022 ruled the agreement should be interpreted as 55 years in international age, citing communication records between unionists.