Time in South Korea: KST, the Colonial Clock, and Why Seoul Uses Japan's Timezone

South Korea uses Korea Standard Time (KST), UTC+9, year-round, with no daylight saving time. The IANA identifier is Asia/Seoul.

Key facts about time in South Korea

• Timezone: Korea Standard Time (KST)
• UTC offset: +09:00
• DST: no
• IANA identifier: Asia/Seoul
• Capital: Seoul

1908: Korea’s first standard time

Before 1908, Korea ran on local mean time tied to the Seoul meridian: 8 hours, 27 minutes, and 52 seconds ahead of UTC, recorded in the IANA database verbatim from that LMT value. On 1 April 1908, the Korean Empire adopted its first formal standard time: UTC+8:30. The source is Official Journal Article No. 3994 (decree No. 5).

This places an important detail that the colonial-time narrative often elides: Korea already had a standard clock, set by Korean decree, before Japan imposed its own.

Seoul sits at approximately 127 degrees East longitude. That corresponds to a natural solar time of roughly UTC+8:28, meaning the 1908 Korean standard of UTC+8:30 was almost exactly aligned with the city’s solar position.

1905 and 1910: Japan’s arrival

Japan and Russia competed for influence over the Korean peninsula through the early 20th century. The Treaty of Portsmouth, signed on 5 September 1905 and mediated by US President Theodore Roosevelt, ended the Russo-Japanese War and recognized Japan’s hegemony over Korea, which became a Japanese protectorate. Japan formally annexed Korea on 29 August 1910.

1912: the clock changes

Two years into formal colonial rule, on 1 January 1912, the Japanese administration imposed Japan Standard Time on Korea. The source is Governor-General of Korea Official Gazette Issue No. 367 (Announcement No. 338). The change moved Korea from UTC+8:30 to UTC+9 to align with Tokyo’s administrative clock, simplifying railway schedules, telegraph communications, and official coordination across Japanese-administered territories.

UTC+9 placed solar noon at approximately 11:00 AM Seoul sun time. The country was running an hour fast relative to its own sky.

For 35 years of Japanese colonial rule, Korean clocks ran on Japanese time.

1945 to 1961: reclamation and reversal

After Japan’s surrender in August 1945 and Korea’s liberation, the country initially remained on UTC+9. The Korean War ran from 1950 to 1953. On 21 March 1954, under President Syngman Rhee, South Korea moved the clocks back to UTC+8:30. The legal instrument was Presidential Decree No. 876, issued 17 March 1954. The offset was closer to Seoul’s natural solar position and was a clear departure from the Japanese-imposed clock.

The move also came with a practical complication: the IANA database notes that during the period when Korea was on UTC+8:30, the international aviation, marine, and meteorological industries in the country refused to follow it and continued using UTC+9 for interoperability.

On 10 August 1961, Park Chung-hee’s newly installed military government moved the clocks forward to UTC+9 under Law No. 676 (enacted 7 August 1961). The stated reasons were alignment with Japan, the country’s largest trading partner, and administrative simplicity.

South Korea has been on UTC+9 ever since.

Daylight saving: a longer history than the Olympics

South Korea’s DST history runs deeper than its brief 1988 appearance. The full record from the IANA database:

• 1948 to 1951: DST observed (suspended during the Korean War, 1950-1953)
• 1955 to 1960: DST resumed
• 1961: DST ended with Korea’s return to UTC+9
• 1987 to 1988: DST reintroduced for two years, coinciding with preparations for and the hosting of the 1988 Seoul Olympics

Since 1988, no DST.

The recurring debate

The question of whether South Korea should move back to UTC+8:30 surfaces periodically. Seoul’s natural solar alignment is closer to UTC+8:28. UTC+9 means the sun rises very early in summer (before 5 AM in some regions), and workers commuting before dawn in winter do so in complete darkness longer than the clock position requires.

South Korea’s largest trading partner is now China, which uses UTC+8. Moving to UTC+8 would align Korea with Beijing without sharing Japan’s clock. The arguments against change are cultural and practical: decades of business integration at UTC+9, embedded train and broadcast schedules, and the friction cost of changing every clock assumption across a fully industrialized economy.

North Korea’s experiment

The debate has a recent parallel across the border. On 15 August 2015, the 70th anniversary of Korea’s liberation from Japan, North Korea moved its clocks to UTC+8:30 and called it Pyongyang Time. The IANA database records the Zone Asia/Pyongyang transition precisely: 9:00 - KST 2015 Aug 15 00:00. North Korean state media framed the move as a rejection of the Japanese-imposed offset.

The experiment lasted less than three years. On 30 April 2018, the Presidium of the Supreme People’s Assembly issued Decree No. 2232, published in the Rodong Sinmun, announcing reversion to UTC+9. The transition happened at 23:30 on 4 May 2018, putting North Korea back in sync with South Korea. The IANA entry: 8:30 - KST 2018 May 4 23:30 / 9:00 - KST. The stated reason was to align clocks with South Korea as a gesture toward inter-Korean unity.

South Korea kept its clock at UTC+9 throughout, unchanged.

For developers

• IANA timezone: Asia/Seoul
• UTC offset: +09:00 year-round
• No DST currently; last observed 1987-1988
• Full DST history: 1948, 1949-1951 (with Korean War suspension), 1955-1960, 1987-1988
• Historical offsets: LMT (8:27:52) until 1908; UTC+8:30 from 1908; UTC+9 (JST) from 1912 Jan 1; UTC+9 from 1945; UTC+8:30 from 1954 Mar 21; UTC+9 from 1961 Aug 10 to present
• North Korea (Asia/Pyongyang): UTC+8:30 from 2015-08-15 to 2018-05-04 23:30; UTC+9 since

Sources

• IANA Time Zone Database, asia file: https://raw.githubusercontent.com/eggert/tz/main/asia (Zone Asia/Seoul, Zone Asia/Pyongyang, Rule ROK lines, and associated comments citing Korean government primary sources)
• Governor-General of Korea Official Gazette Issue No. 367, Announcement No. 338 (1912) - cited in IANA asia file comments
• Presidential Decree No. 876 (1954-03-17) - cited in IANA asia file comments
• Law No. 676 (1961-08-07) - cited in IANA asia file comments
• Rodong Sinmun 2018-04-30, Decree No. 2232 - cited in IANA asia file comments; NK News archive: https://www.nknews.org/kcna/wp-content/uploads/sites/5/2018/04/rodong-2018-04-30.pdf
• Treaty of Portsmouth (1905): https://en.wikipedia.org/wiki/Treaty_of_Portsmouth
• Korea under Japanese rule: https://en.wikipedia.org/wiki/Korea_under_Japanese_rule