Time zones explained

Why the world is split into time zones, how UTC and GMT fit in, and why some places are 30 or 45 minutes off the hour.

By timenow.click · Updated 19 October 2026 · 6 min read

If you have ever woken someone up with a phone call, or missed a meeting because it was “9 a.m. their time”, you have run into time zones. They are one of those systems everyone uses and few people think about. This guide explains how they work and why they are drawn the way they are.

Why we have time zones at all

For most of history, every town kept its own time, based on the sun. Noon was when the sun was highest in the sky, so a town a little to the west had a slightly later noon than its neighbour. That was fine when travel was slow. It stopped working when railways and the telegraph arrived, because timetables had to say when a train would leave, and every station had a different clock.

The solution was standard time: agreeing that everyone in a broad region would use the same clock, even if it didn't exactly match the sun overhead. British railways moved to London time in the 1840s. In North America the railways switched to a handful of standard zones on 18 November 1883. A year later, delegates at the International Meridian Conference in Washington chose the meridian through Greenwich, in London, as the world's starting line for longitude.

Why each zone is roughly 15 degrees wide

The Earth turns 360 degrees in 24 hours, which is 15 degrees every hour. So in theory the world divides neatly into 24 zones, each 15 degrees of longitude wide and one hour apart, starting from Greenwich at 0°. Move 15 degrees east and the clock goes forward an hour; move 15 degrees west and it goes back an hour.

In practice the lines on a time zone map wiggle all over the place, because countries choose their own time. Governments usually want a whole country, or at least a whole state or province, on one clock, and they often prefer to share a time with their main trading partners.

Where the map and the clock disagree

China is the best-known example. It stretches across about five “natural” time zones but uses a single time, UTC+8, everywhere. In the far west of the country the sun can be overhead closer to 3 p.m. than to noon. Spain is another: almost all of it lies west of Greenwich, yet since 1940 it has used the same time as Germany. At the other extreme, Russia uses eleven time zones, from Kaliningrad on the Baltic to Kamchatka on the Pacific.

UTC and GMT: what's the difference?

You will see time zones written as offsets from UTC, such as UTC+1 or UTC−5. UTC stands for Coordinated Universal Time. It was introduced in the 1960s and is kept by a network of atomic clocks around the world, which makes it far steadier than any time based on the Earth's rotation.

GMT, Greenwich Mean Time, is the older standard, originally worked out from the sun's position over Greenwich. For everyday purposes GMT and UTC are the same time. The difference is that UTC is the scientific standard, while GMT is now mainly used as the name of the UK's winter time zone. In summer the UK switches to British Summer Time, which is UTC+1.

Because the Earth's rotation is slightly irregular, UTC has occasionally had a “leap second” added to keep it close to solar time. In 2022 the international body responsible for measurement standards voted to stop adding leap seconds by 2035.

Half-hour and quarter-hour time zones

Not every place is a whole number of hours from UTC. Some countries chose a time that fits their geography better than the nearest whole hour. India, for example, sets its clocks by the meridian at 82.5° east, and 82.5 divided by 15 is 5.5, giving UTC+5:30. Nepal went a step further and uses UTC+5:45, partly to keep its own time separate from India's.

Here are some of the unusual offsets in use right now:

The International Date Line

If you keep moving east, adding an hour every 15 degrees, you eventually have to drop a whole day somewhere. That place is the International Date Line, which runs roughly down the 180° meridian in the middle of the Pacific. Cross it heading west and you skip forward a day; cross it heading east and you repeat one.

Like time zone boundaries, the date line bends around countries that want to be on a particular side. In 1995 Kiribati moved it so that all of its islands shared the same date, which is why its Line Islands are at UTC+14, the earliest time on Earth. In 2011 Samoa jumped from one side to the other to line up with Australia and New Zealand, skipping Friday 30 December entirely.

Daylight saving time

Many countries move their clocks forward an hour in summer and back again in winter. The idea is to shift an hour of daylight from the early morning, when many people are asleep, to the evening. Most of Europe and North America does this, but most of Asia, Africa and South America does not, and the start and end dates vary from country to country.

That variation is a common source of confusion. The UK and the US, for example, change their clocks on different weekends, so for a few weeks each spring and autumn the usual five-hour gap between London and New York becomes four. Countries in the southern hemisphere, such as Australia and New Zealand, have their summer time from around October to April, the opposite of Europe.

How computers keep track of it all

Phones and computers don't store time zones as simple offsets, because offsets change with daylight saving and with new laws. Instead they use the IANA time zone database, a public record of every region's clock rules and history, with names such as Europe/London, America/New_York and Asia/Kolkata. When a government changes its rules, the database is updated and the change reaches your devices through software updates. timenow.click uses the same database, built into your browser.

Tips for talking about time across zones

• Name a city, not an abbreviation. “CST” can mean Central Standard Time in the US, China Standard Time or Cuba Standard Time, and “IST” can mean India, Ireland or Israel. “3 p.m. Chicago time” is unambiguous.
• Give both times. “10:00 London / 18:00 Singapore” saves the other person a calculation and catches mistakes early.
• Watch the date. A late-evening meeting in New York is the next morning in Asia. Say the day as well as the time.
• Check again around clock changes. In March, April, October and November the gaps between countries can shift by an hour for a week or two.

To check any two places quickly, use Compare two places on the home page, which shows the live difference and converts any time from one city to another.