Time zones explained
When it is noon in Amsterdam, people in Tokyo are already sitting down to dinner, and most New Yorkers are still asleep. Time zones allow clocks worldwide to align with the position of the sun — an invention made indispensable by railways and telegraphy in the nineteenth century.
Why the Earth needs time zones
The Earth rotates once in 24 hours, covering 360 degrees. That means it travels 360 ÷ 24 = 15 degrees per hour. This principle forms the basis of the time zone system: every 15 degrees of longitude, solar time shifts by exactly one hour. In theory, this gives 24 equal zones.
Before 1884, almost every city used its own local time derived from the highest position of the sun. That worked as long as the stagecoach was the fastest form of transport. With the rise of railways, travellers had to reset their watches at every station. In 1884, 26 countries gathered at the International Meridian Conference in Washington and agreed to take the meridian through Greenwich (London) as the zero point. Since then, UTC — the successor to GMT — has been the global reference. More about UTC and the prime meridian is on the page UTC, GMT and the prime meridian.
The 15-degree rule yields 24 zones, but reality is messier. Countries choose zones for political and economic reasons, not purely on the basis of longitude. As a result, in 2025 there are approximately 38 unique UTC offsets in use worldwide, including zones with a half-hour (UTC+5:30) or even a quarter-hour (UTC+5:45) offset (source: IANA Time Zone Database).
UTC as the global standard
UTC stands for Coordinated Universal Time and is the international time standard maintained by a network of atomic clocks around the world. Atomic clocks measure time based on the oscillation frequency of caesium atoms — more precise than Earth's rotation, which fluctuates slightly. To prevent the difference from growing too large, leap seconds are occasionally inserted. The Royal Observatory Greenwich maintained the official time standard until 1967; after that UTC took over the role, although Greenwich still marks the 0° meridian.
A UTC offset is written as UTC+X or UTC−X: a positive offset means the local clock is ahead of UTC; a negative means it is behind. The Netherlands uses UTC+1 in winter (Central European Time) and UTC+2 in summer (Central European Summer Time). More about the annual clock change is on the page daylight saving time around the world.
Special cases: China, India and Nepal
Not every country follows the 15-degree logic neatly. Three notable cases:
China (UTC+8) — The country spans five time zones geographically, from UTC+6 in the west (Xinjiang) to UTC+8 in the east (Shanghai). In 1949 the newly formed People's Republic decided to put the entire country on Beijing time (UTC+8). The result: in Ürümqi in the far west the sun does not rise until around 10:00, and people stay awake long past midnight when darkness falls after 22:00. Asia has more time zone variation than any other continent as a result of such choices.
India (UTC+5:30) — India chose a half-hour offset as a compromise. At independence in 1947 the subcontinent had several local times; a single half-zone kept solar time acceptable everywhere without disadvantaging the far east or west.
Nepal (UTC+5:45) — Nepal goes one step further with a quarter-hour offset, UTC+5:45. This is the only UTC+:45 offset in use worldwide. Nepal wished to maintain distance from both India and China, and the 5:45 choice enshrined its own identity in the clock.
For more special borders and geographical extremities, see also the page about the equator and hemispheres, which explains how latitude and longitude divide the world.
Live clock — six cities around the world
The clock below shows the current time in six cities on different continents, calculated directly by your device. You can see at a glance how far apart the clocks are.
Live times based on your device · Source: IANA Time Zone Database
Worked example — 12:00 in the Netherlands
Suppose it is exactly 12:00 in the Netherlands (UTC+1 in winter):
• Paris (UTC+1): also 12:00 — same zone.
• Lagos (UTC+1): also 12:00 — West Africa uses the same offset.
• New Delhi (UTC+5:30): 16:30 — 4.5 hours ahead of the Netherlands.
• Tokyo (UTC+9): 20:00 — evening has already fallen.
• Sydney (UTC+11 in summer): 22:00 — nearly bedtime.
• New York (UTC−5): 06:00 — morning is just beginning.
The International Date Line
If you travel far enough east or west, you reach a point where the clock does not jump one hour but an entire day. This line runs roughly along the 180° meridian in the Pacific Ocean. Because it curves around inhabited islands, it sometimes shifts the date from one side to the other. Everything about the date change is on the page the International Date Line.
Frequently asked questions
How many time zones are there on Earth?
Officially there are 24 main zones based on 15° segments. In practice there are approximately 38 unique UTC offsets, due to half-hour and quarter-hour shifts and political boundaries (source: IANA Time Zone Database).
Why does China use only one time zone?
China decided in 1949 to put the entire country on UTC+8 for political unity, even though the country geographically spans five 15° zones. In western China the clock is therefore far ahead of solar time.
What is the difference between UTC and GMT?
UTC is the international standard based on atomic clocks. GMT is a time zone based on the position of the sun above Greenwich. They differ by at most 0.9 seconds, but UTC is the scientific standard. More detail on UTC, GMT and the prime meridian.
When do the clocks change in Europe?
In the EU the clocks go forward on the last Sunday of March (summer time) and back on the last Sunday of October. Read more on the page daylight saving time.
Sources
- IANA Time Zone Database — tz.iana.org (time zone offsets and names)
- Royal Observatory Greenwich — the historical zero point for GMT
- International Meridian Conference, Washington 1884 — establishment of the prime meridian
- CIA World Factbook — country data and time zone information