Why time zones exist
The Earth turns once every 24 hours, or 15 degrees an hour. The moment the sun is highest therefore depends on longitude, and towns once kept their own local time. When railways and the telegraph linked distant places quickly, those differences caused confusion, and from the late nineteenth century a system took hold of choosing a reference meridian and dividing time into regular bands. In theory that gives 24 zones, one per 15 degrees, but real boundaries wind along national and administrative borders.
UTC and Greenwich Mean Time
Coordinated Universal Time, or UTC, is the reference for world time. It is based on extremely precise atomic time but has been kept within 0.9 seconds of the Earth's rotation by inserting leap seconds when needed. Since 1972, 27 leap seconds have been added, the most recent at the end of 2016. In 2022 the General Conference on Weights and Measures decided to abolish leap seconds by 2035 at the latest. Greenwich Mean Time (GMT) is the older reference based on Greenwich in England and also the name of the UK's winter time; in everyday use it is treated as almost the same as UTC, though strictly they are different concepts.
What UTC+9 means
Local time is expressed as how many hours are added to or subtracted from UTC. Korea Standard Time is UTC+9, so when it is midnight UTC it is 9 a.m. in Seoul. Japan uses the same UTC+9. Not every zone falls on a whole hour.
- India: UTC+5:30
- Nepal: UTC+5:45
- Newfoundland, Canada: UTC−3:30
- Earliest: some islands of Kiribati at UTC+14
- Latest: some uninhabited islands at UTC−12
- China is vast but uses a single zone, UTC+8
The trap of daylight saving time
The US, Canada and many European countries move their clocks forward an hour in summer. Currently the US usually runs it from the second Sunday in March to the first Sunday in November and the EU from the last Sunday in March to the last Sunday in October, but rules can change, so check official sources. As a result the gap between Seoul and London is nine hours in winter and eight in summer, and between Seoul and New York fourteen in winter and thirteen in summer. In the southern hemisphere the seasons are reversed, so daylight saving falls in the opposite months. Because the US and Europe switch on different dates, for a few weeks a year the usual gap between them changes. Korea itself used daylight saving time in 1987 and 1988.
The date line and arriving 'yesterday'
Near 180 degrees longitude lies the International Date Line. Crossing it eastwards turns the date back a day; crossing westwards adds one. That is why a flight from Korea to the US can land at an earlier clock time than it left, sometimes on the previous date. Remembering that times on a ticket are always the local time of each airport makes this less confusing. To work out the actual journey time, convert both times to UTC and subtract.
Notation that avoids confusion
The international standard ISO 8601 writes date and time from the largest unit down and appends the offset from UTC. For example, 2026-10-01T09:00+09:00 is 9 a.m. in Seoul, which in UTC is 2026-10-01T00:00Z; the trailing Z means UTC. Unix time, used by computers, counts seconds since midnight UTC on 1 January 1970, so it is the same number everywhere on Earth, and it does not count leap seconds. A date written as 01/10/2026 reads as 10 January in some countries and 1 October in others, so avoid it in international documents.
Arranging times across zones
When arranging something with a person in another zone, saying 'tomorrow at 10 a.m.' immediately raises the question of whose tomorrow. Writing it down like this removes almost all misunderstanding.
- Put the city name or UTC offset next to the time
- Give both local times side by side (9 a.m. Seoul = midnight London, the day before)
- Midnight is ambiguous between 0:00 and 24:00, so write 23:59 or 0:01
- Recheck recurring meetings in the week the other side switches daylight saving
Common mistakes
Most time-difference errors come from forgetting the date or the season. Memorising a single number means you will be wrong when the season changes.
- Assuming the time difference stays the same all year
- Changing the time but missing that the date has rolled over
- Treating GMT and UK local time as equal in summer
- Sending a time with no zone attached
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