A Guide to the World's Time Zones

In theory, the planet is split into 24 neat one-hour slices. In practice, there are 38 distinct offsets in everyday use, and the boundaries between them are drawn by national parliaments rather than meridians. Here's how the system actually works.

Why time zones exist at all

Before the railways, every town set its clock by the position of the sun overhead. When it was noon in London, it was 11:51 AM in Bristol. Nobody minded — you set your watch when you arrived.

Long-distance trains made that intolerable. A railway timetable assumes a single shared time. Britain adopted Greenwich Mean Time across all stations in 1847. The United States and Canada followed in 1883, dividing the continent into four standard zones agreed between the major railway companies.

The current global system was formalised at the International Meridian Conference in Washington in 1884. Twenty-six nations agreed that the prime meridian would pass through Greenwich, that the day would begin at midnight there, and that the world would be divided into 24 zones each one hour wide and 15 degrees of longitude apart.

The 24 standard zones

The standard zones run from UTC−12 in the middle of the Pacific to UTC+12 at the international date line. Each zone is theoretically 15 degrees of longitude wide. Walking due east, every 15 degrees you cross adds one hour. Walking due west subtracts one.

That's the geometry. The actual map looks nothing like that.

The half-hour and quarter-hour offsets

Several countries deliberately sit between the standard one-hour grid:

• India — UTC+5:30, a single time zone for the entire country, half an hour ahead of Pakistan and half an hour behind Bangladesh.
• Iran — UTC+3:30 standard, UTC+4:30 in summer.
• Afghanistan — UTC+4:30.
• Myanmar (Burma) — UTC+6:30.
• Sri Lanka — UTC+5:30.
• Newfoundland (Canada) — UTC−3:30, an artefact of the colony's pre-1949 history as an independent dominion.
• Marquesas Islands — UTC−9:30.
• Central Australia (Northern Territory and South Australia) — UTC+9:30.

Three places use quarter-hour offsets:

• Nepal — UTC+5:45, set at the longitude of Mount Gauri Shankar in 1986 specifically to differentiate Nepal from neighbouring India.
• Chatham Islands (New Zealand) — UTC+12:45.
• Eucla (Western Australia) — UTC+8:45, an unofficial local arrangement for a few hundred residents.

The political map

Time zones are decided by governments, and governments make political decisions:

China — one zone for 5,000 km

The People's Republic of China spans five geographical time zones but uses a single official zone, Beijing Time (UTC+8). In Kashgar, on China's western edge, "Beijing Time" noon happens around 3 PM solar time. Many locals quietly run on an unofficial UTC+6 alongside the official clock.

Russia — eleven zones

Russia spans 11 time zones from Kaliningrad on the Baltic to Kamchatka on the Pacific — the widest east–west extent of any country. Russia abolished daylight saving in 2011 and now keeps all 11 zones on permanent standard time.

The Pacific date line gymnastics

Samoa jumped across the date line in December 2011, skipping 30 December entirely. The country had been on UTC−11 to align with the United States; switching to UTC+13 aligned it with Australia and New Zealand, its main trading partners. Kiribati made a similar jump in 1995, creating its UTC+14 zone — the easternmost civil time on Earth.

Spain on the wrong clock

Spain sits roughly on the same longitude as the United Kingdom but uses Central European Time (UTC+1), one hour ahead. The arrangement dates to 1940, when the Franco government aligned Spanish clocks with Berlin. It was never reversed. Spaniards now eat lunch at 2 PM and dinner at 10 PM partly because their clocks are an hour off the sun.

Venezuela's half-hour move

Venezuela switched from UTC−4 to UTC−4:30 in 2007 under Hugo Chávez, then back to UTC−4 in 2016 under Nicolás Maduro to save electricity. A symbolic move, twice over.

The IANA Time Zone Database

If you have ever set a phone, used a calendar app, or written software that handles dates, you have used the IANA Time Zone Database — informally known as "tz database" or "zoneinfo." It is the canonical machine-readable record of every time zone, every offset, every DST rule, and every historical change since the introduction of standard time in the 19th century.

Each zone is identified by a Region/City name like Europe/London, America/New_York, or Asia/Kolkata. The city is chosen to be a representative population centre with stable historical rules — which is why the Indian time zone is Asia/Kolkata rather than the politically obvious Asia/Mumbai (Kolkata had the more consistent historical record at the time the database was compiled).

The database is maintained by volunteers under the auspices of IANA, with updates released several times a year whenever a country changes its rules. Operating systems, browsers, and programming languages all pull from the same source.

How "your" time zone is detected

When a website displays your local time, it isn't asking your location. It is reading the time zone setting your operating system has already chosen — usually from your IP address at install time, or from the system locale. The browser exposes that name (e.g. Europe/Paris) through the Intl.DateTimeFormat JavaScript API.

This is exactly how The Pocket Clock figures out your timezone without ever asking. The detection happens entirely on your device, and the result never leaves it.

The bottom line

The world's time zones are a 140-year-old compromise between the geometry of the Earth and the politics of the people on it. Twenty-four neat slices was always a fiction; the reality is 38 offsets, 11 Russian zones, a Chinese mega-zone five hours wide, and one Nepali parliament that picked a quarter-hour offset to make a point. The IANA database is the only thing keeping the whole patchwork legible to computers.