Does midnight belong to today or tomorrow?

According to a counseling case from Korea's National Institute of Korean Language, midnight exactly, 24:00, belongs to the previous day, and from 00:01 a new day begins. That minute is correctly called ‘ojeon 0-si 1-bun’ (오전 0시 1분, ‘0:01 a.m.’), not ‘ojeon 12-si 1-bun’ (오전 12시 1분, ‘12:01 a.m.’).

As in ‘I got home after midnight,’ Koreans usually feel midnight as the end of a day. Still, there is no fixed rule. The Institute's Online 가나다 service said ‘naeil jajeong’ (내일 자정, ‘midnight tomorrow’) can differ by a whole day depending on whether midnight is seen as a day's start or end, so it is hard to say for sure.

The U.S. National Institute of Standards and Technology (NIST) says 12 a.m. and 12 p.m. are ambiguous and should not be used. A.m. means before noon and p.m. after noon, but noon is neither, and midnight is both twelve hours before and twelve hours after noon.

The Chicago Manual of Style, an American publishing guide, also says that without context we cannot know which day 12:00 a.m. refers to. It recommends giving both dates, as in ‘midnight, April 12–13.’

Phones and computers on the 12-hour clock usually show midnight as 12 a.m. That makes it easy to assume it is the correct answer, but the Chicago Manual treats it as a convenience. The National Institute of Korean Language likewise says noon and midnight divide a.m. from p.m., so neither fits one side.

For important times such as contracts or deadlines, moving one minute away, to 11:59 p.m. or 12:01 a.m., as NIST suggests, keeps things clean. Another option is the 24-hour clock with the date attached, such as ‘24:00 on the 13th’ or ‘00:00 on the 14th.’

If a notice says ‘by midnight on the 13th,’ finishing before 11:59 p.m. on the 13th is the safer choice. When making plans with friends, saying ‘midnight between the 13th and the 14th’ avoids any mix-up.