July 3, 2026 10 min read

How Time Zones Work

A time zone is a region where everyone keeps the same legal clock time, and that time is defined as an offset from Coordinated Universal Time (UTC). New Delhi is UTC+5:30 all year; New York is UTC-5 in winter and UTC-4 in summer. Governments pick the offsets, which is why the map is jagged and why some zones sit on the half hour.

What a time zone is

A time zone is an area that runs its clocks to one legal time, written as an offset from UTC, the reference time scale kept by atomic clocks. UTC+1 means one hour ahead of UTC; UTC-5 means five hours behind. When it is 12:00 UTC, it is 13:00 in a UTC+1 zone and 07:00 in a UTC-5 zone.

Two details catch people out. First, an offset belongs to a place on a given date, not to the place forever: London is UTC+0 in January and UTC+1 in July. Second, a zone exists because a government says so. The Sun decides when local noon happens, but the law decides what the clocks show.

You can see both at work on any city page. The current time in London shows today's offset and the next clock change, and the UTC page shows the reference everyone else is measured from.

From railway time to standard zones

Until the middle of the 1800s, every town kept its own local mean time, set by the Sun. Bristol, about 2.6 degrees west of London, ran roughly ten minutes behind it. Nobody minded while travel took days. Trains made the difference visible, because a timetable cannot work with a different "noon" at every station.

British railways fixed it first. The Great Western Railway adopted London time in November 1840, most lines used it by 1847, and the law caught up when the Statutes (Definition of Time) Act received Royal Assent on August 2, 1880, making Greenwich Mean Time the legal time of Great Britain.

North America followed with a larger problem: dozens of railroad times across a continent. On Sunday, November 18, 1883, US and Canadian railroads switched to standard zones based on Greenwich, a date remembered as "the day of two noons" because some towns observed noon twice. Federal law arrived with the Standard Time Act of March 19, 1918. Its modern form, 15 U.S.C. § 261, divides the United States into nine zones, from UTC-4 (Puerto Rico) to UTC-11 (American Samoa), plus UTC+10 for Guam and the Northern Mariana Islands.

The world still needed a common zero line. In October 1884, the International Meridian Conference in Washington, D.C. voted to make the Greenwich meridian the prime meridian. Greenwich won largely because nearly two thirds of the world's ships already used charts based on it, so it caused the least disruption.

Reading a UTC offset

An offset is the amount you add to UTC to get local time. UTC+5:30 means "UTC plus five and a half hours". UTC-3:30 means "UTC minus three and a half hours". To convert between two places, you only need the difference between their offsets on that date.

1. Look up both offsets for the date in question. City pages show the current one, and the converter handles future dates.
2. Subtract the source offset from the target offset. That is the gap.
3. Add the gap to the source time. If the result passes midnight, move the date forward (or back, for a negative gap).

Worked example: a call at 9:00 a.m. in London, to someone in New Delhi. In July, London is on British Summer Time, UTC+1. New Delhi is on India Standard Time, UTC+5:30, all year. The gap is 5:30 minus 1:00, which is 4 hours 30 minutes, so 9:00 in London is 13:30 in Delhi. In January the gap grows to 5:30, because London drops back to UTC+0.

Negative offsets work the same way. St. John's, Newfoundland, is UTC-3:30 in winter, so 12:00 UTC is 08:30 there. Going the other way, subtract the offset to get back to UTC: 18:00 in Delhi minus 5:30 is 12:30 UTC. If you would rather not do the arithmetic, convert 9:00 London time to Delhi in our time zone converter and add as many cities as you like.

Why zones do not follow straight lines

The tidy version goes like this: the Earth turns 360 degrees in 24 hours, so each hour of offset covers 15 degrees of longitude. Ships at sea still use those 15-degree "nautical" bands. On land, borders, trade and politics win, and three cases show how far a zone can drift from the Sun.

China: one clock for 60 degrees of longitude

The whole of China officially runs on Beijing Time, UTC+8, although the country stretches from about 73 degrees east to 135 degrees east. That is wide enough for four solar zones. In Kashgar, near 76 degrees east, the Sun is highest close to 3 p.m. by the clock. The single China time zone has a whole story behind it.

Spain: on Berlin's clock

Most of Spain lies west of the Greenwich meridian, which on longitude alone would put it on UTC+0 with Portugal and the UK. Instead, Madrid has used Central European Time, UTC+1, since March 1940, the same as Berlin and Warsaw. In summer the Sun is not highest over Madrid until after 2 p.m. by the clock.

Kiribati: moving the date line

Kiribati is a string of islands across the central Pacific. Its capital, Tarawa, sat at UTC+12 while its eastern Line Islands sat at UTC-10, 22 hours apart and usually on different calendar days. At the end of 1994 the Line Islands moved to UTC+14. According to the tz database, Kiritimati went from Friday, December 30, 1994 straight to Sunday, January 1, 1995. That is why the earliest clocks on Earth are in Kiribati. Samoa made a similar jump in 2011, skipping Friday, December 30. The International Date Line bends around both.

How many time zones are there?

In theory there are 24, one per hour. In practice there are more distinct offsets than that. We counted the offsets used by every zone in the IANA tz database's main table: on July 3, 2026 there are 37 different offsets in use, and 39 across a full year, because Newfoundland and the Chatham Islands move between unusual values when their clocks change.

The offsets people live by run from UTC-11 (American Samoa, Niue) to UTC+14 (Kiritimati). No inhabited place keeps UTC-12; it survives only as the nautical time for ships just east of the date line. That 25-hour spread has an odd result: between 10:00 and 11:00 UTC every day, three calendar dates are in use at once.

About a dozen offsets are not whole hours. The 30-minute ones are the best known (India is the largest country on one), and a few are 45 minutes. Our guide to half-hour and 45-minute time zones covers how each came about.

You may also see figures in the hundreds. Those count tz database identifiers, not offsets. The database keeps a separate ID for every region whose clocks have differed at any point since 1970, so places that agree today can still have their own entries. Its main table alone lists more than 300. The IANA time zone database explains the naming, and it is worth knowing that IANA records what governments decide; it does not set anyone's time. Its own documentation says users who need authoritative data should consult national standards bodies.

Time zones and daylight saving time

Daylight saving time changes a zone's offset, not the zone. New York stays in the America/New_York zone all year, but its offset moves from UTC-5 (EST) to UTC-4 (EDT) on the second Sunday in March and back on the first Sunday in November. In 2026 those dates are March 8 and November 1. London switches on the last Sundays of March and October, which this year are March 29 and October 25.

Because the two schedules differ, the New York to London gap is five hours for most of the year. It was four hours from March 8 to March 29, and it will be four hours again from October 25 to November 1. The southern hemisphere runs the other way: Sydney moves to UTC+11 on October 4, 2026, just as the northern hemisphere prepares to fall back. Meetings that cross those weeks are where most scheduling mistakes happen.

Abbreviations add their own confusion. CST can mean Central Standard Time or China Standard Time, and IST is used in India, Ireland and Israel. The tz project itself recommends numeric offsets such as -0600 over abbreviations. Our list of time zone abbreviations flags the clashes, and EST vs EDT covers the pair people mix up most.

Find the time anywhere

For one place right now, open its city page; each one shows the local time, the offset, the IANA zone and the next clock change, such as Kathmandu at UTC+5:45. For several places at once, the world clock keeps a list of cities ticking side by side. For a meeting or a flight, use the time zone converter with the actual date, so daylight saving is taken into account.

For regional detail, start with US time zones, Europe's time zones or India Standard Time, and check recent time zone changes before relying on an old rule. The time zone directory lists every zone and abbreviation we track.

Every conversion comes down to two offsets and one subtraction. Pick the next call or trip on your calendar and run it through the converter with its real date.

Frequently asked questions