Why does Korea share Japan's clock? The reason Seoul's true noon is around 12:30

BOTH Chochim editorial team · November 6, 2026

Written as of November 6, 2026. The current rule follows Korea's Standard Time Act (as published by the Korea Legislation Research Institute); the history of standard time and daylight saving follows the National Archives of Korea; time keeping follows KRISS; Japan Standard Time follows NICT and the Japan Science and Technology Agency. North Korea's time zone changes are based on major news reports, as we could not check the official announcement directly. The detailed reasons for the 1961 change are only summarized by the National Archives, so we do not speculate on them.

You land in Tokyo, switch your phone back on, and the time hasn't changed. No jet lag math, which is nice. But then it seems odd. Tokyo is well to the east of Seoul on the map, and the sun must rise there earlier. So why do the clocks match?

The answer is written into law. Korea's Standard Time Act says standard time is set using the meridian of 135 degrees east longitude. That line never crosses Korean soil; it runs through Akashi in Hyogo, Japan. In effect, Korea lives by a meridian on Japanese ground. Here is how that happened, and how it leaves Korea's day slightly out of step with the sun.

15 degrees per hour: how standard time works

The Earth turns 360 degrees in 24 hours. That works out to one hour for every 15 degrees of longitude, or four minutes per degree. Solar noon, when the sun is highest, arrives an hour earlier for every 15 degrees you go east.

If every town set its clocks to its own noon, neighbors would be minutes apart, and train timetables and broadcast schedules would be impossible. So each country picks one "standard meridian" and everyone uses its time. That is standard time.

Standard meridians are usually multiples of 15, so they sit a whole number of hours from Greenwich (0°). 135°E is 15 × 9, which makes it exactly nine hours ahead of Coordinated Universal Time (UTC). The National Archives of Korea describes today's Korean standard time as "UTC plus nine hours, at 135°E."

The catch is geography. According to the National Archives, Korea lies between 124 and 132 degrees east. Picking a multiple of 15 means choosing between 120°E (UTC+8) and 135°E (UTC+9). The midpoint, 127.5°E, gives UTC+8:30. Korea has actually used 127.5°E, not once but twice.

Korean standard time has changed four times

The National Archives lists four changes:

April 1, 1908: the Korean Empire adopts 127.5°E (UTC+8:30)
January 1, 1912: under Japanese colonial rule, the standard moves to 135°E, the same as Japan (UTC+9)
March 21, 1954: back to 127.5°E
August 10, 1961: back to 135°E, which is still in use

The 1954 change is well documented. The Ministry of Education brought a proposal to the State Council, arguing that the 135° standard left the clock 30 minutes off from reality. After cabinet approval, Presidential Decree No. 876 on changing the standard meridian appeared in the official gazette on March 17, 1954, and 12:30 a.m. on March 21 became the new midnight. Overnight, clocks went back half an hour.

Seven years later it flipped again. According to the National Archives, the Ministry of National Defense proposed a return to 135°E in July 1960, and after cabinet approval a law changing the standard meridian was enacted on August 7, 1961, taking effect on August 10. This time clocks jumped forward 30 minutes.

The standard has not changed in the 65 years since. The law was wholly rewritten in 2011 as the Standard Time Act, but the rule is the same. So what trace has that 30-minute offset left on daily life?

The sun peaks over Seoul around 12:30

Here is the math. Seoul sits at roughly 127°E, eight degrees west of 135°E. At four minutes per degree, that's about 32 minutes. The sun reaches its highest point over Seoul a little more than half an hour after it does over Akashi.

The National Archives puts it plainly: noon on Korean clocks is not true noon; 12:30 is the real noon. When the lunch bell rings, the sun is still climbing. Because of the shape of Earth's orbit, the exact time shifts a little with the seasons, but on average it runs about half an hour late.

The gap varies across the country. Using the National Archives' range of 124° to 132°E, the western edge reaches solar noon about 44 minutes after the clock says 12, and the eastern edge about 12 minutes after. Same clock time, quite different sun heights on islands in the Yellow Sea and the East Sea.

And here is a twist: this "late noon" was once seen as something useful. That brings us to daylight saving time.

What daylight saving and "Pyongyang time" reveal

Daylight saving time moves clocks an hour forward in summer to shift daylight into the evening. According to the National Archives, Korea used it from 1949 to 1961, usually from May to September. The 1954 Ministry of Education argument fits here: a 135° standard already acts like clocks set 30 minutes ahead, which undercut the point of summer time.

When the standard returned to 135°E in 1961, daylight saving was dropped, on the grounds that frequent clock changes caused confusion. It came back in 1987 and 1988, ahead of the 1988 Seoul Olympics, to align better with the US and other major countries, and the National Archives says it was abolished in 1989. The current law still allows daylight saving by presidential decree, but it is not in use.

Standard time is not only a technical matter. From August 15, 2015, the 70th anniversary of liberation from Japan, North Korea used "Pyongyang time," 30 minutes behind Seoul (UTC+8:30), the same offset the Korean Empire chose in 1908. After the 2018 inter-Korean summit, The Korea Times and NPR reported that North Korea returned to UTC+9 from May 5, 2018. One meridian, two Koreas, two different decisions.

Chochim checkpoint The 135° meridian only decides what we call the time; it says nothing about how accurate a clock is. Accuracy comes from UTC(KRIS), the time scale KRISS generates with atomic clocks, plus nine hours. For ticketing or course registration, what matters is not 135° versus 127.5°, but how many seconds your clock is off from the ticketing site's server. When reading a foreign release time, just remember Korea is UTC+9.

Frequently asked questions

What does Korean law say about standard time?

The Standard Time Act says standard time "shall be determined with the meridian of 135 degrees east longitude set as the standard meridian," and allows daylight saving time by presidential decree. It is Act No. 10640, wholly amended on May 19, 2011.

Why does Korea use the same time zone as Japan?

The standard moved to Japan's 135°E in 1912 under colonial rule, returned to 127.5°E in 1954, and switched back to 135°E in 1961, where it has stayed. 135°E is exactly nine hours ahead of UTC.

When is true solar noon in Seoul?

The National Archives says "12:30 is the real noon." Seoul is about 127°E, eight degrees west of 135°E, so the sun peaks about 32 minutes later, varying a little with the seasons.

Has Korea ever used daylight saving time?

Yes. According to the National Archives, from 1949 to 1961, and again in 1987 and 1988 ahead of the Seoul Olympics. It is not used today.

In short, Korea sharing Japan's clock is not an accident but the result of decisions made in 1912 and 1961. The price is that Koreans eat lunch about half an hour before the sun reaches its peak.

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