The Earth turns once a day, so noon can't happen everywhere at once — which is why we split the planet into time zones. Underneath all of them sits a single reference clock, UTC, and every zone is just a number of hours ahead of or behind it. Here's how that works, where GMT fits in, why the clocks jump twice a year, and what happens when you cross the date line.
Every zone is an offset from UTC — like UTC+1 or UTC−5.
UTC (Coordinated Universal Time) is the world's reference clock, the same everywhere and never shifted for daylight saving. A zone written UTC+9 is nine hours ahead of it; UTC−5 is five hours behind. Offsets run all the way from UTC−12 to UTC+14.
A few zones, and their offset from UTC
Here are some widely-used zones and how far each sits from UTC. These are standard-time offsets (winter); places that observe daylight saving move an hour for part of the year, which is why some rows note a summer value too.
Offset
Zone
Where
UTC+0
GMT / WET
London & Lisbon (winter; +1 in summer), Reykjavík, Accra
UTC+1
Central European Time
Paris, Berlin, Rome, Madrid (winter; +2 in summer)
UTC+2
Eastern European Time
Athens, Helsinki, Cairo (winter; +3 for some in summer)
UTC+3
Moscow / East Africa
Moscow, Nairobi, Riyadh (no daylight saving)
UTC+4
Gulf Standard Time
Dubai, Abu Dhabi (no daylight saving)
UTC+5:30
India Standard Time
All of India & Sri Lanka — one half-hour zone (no DST)
UTC+5:45
Nepal Time
Nepal — the famous quarter-hour offset
UTC+8
China / Singapore
All of China on one zone; also Singapore, Perth (no DST)
UTC+9
Japan / Korea
Tokyo, Seoul (no daylight saving)
UTC+10
Australian Eastern
Sydney, Melbourne (winter; +11 in summer)
UTC+12
New Zealand
Auckland (winter; +13 in summer), Fiji
UTC−3
Argentina / Brazil east
Buenos Aires, São Paulo (no daylight saving)
UTC−5
US & Canada Eastern
New York, Toronto (winter; −4 in summer)
UTC−6
US & Canada Central
Chicago, Mexico City (winter; −5 in summer for the US)
UTC−8
US & Canada Pacific
Los Angeles, Vancouver (winter; −7 in summer)
A snapshot, not the full list — there are about 38 distinct offsets in real use. Because of daylight saving, the gap between any two of these places can change by an hour twice a year, so always check the live difference before booking a call.
What UTC actually is
UTC — Coordinated Universal Time — is the single clock the whole world measures against. It doesn't belong to any country or city, it isn't shifted for summer, and it's kept by a network of atomic clocks, so it's extraordinarily precise. When you read "the launch is at 18:00 UTC," everyone on Earth can work out their local time from that one number by adding or subtracting their offset.
Think of it as sea level for clocks: every local time is measured as a height above or below it. Tokyo runs at UTC+9, New York at UTC−5 in winter, and the difference between them — fourteen hours — is just the gap between their two offsets.
GMT vs UTC — are they the same?
In everyday speech, yes: both mean "the time at zero offset." The distinction is technical. GMT (Greenwich Mean Time) is a time zone, originally based on the average position of the sun over the Royal Observatory in Greenwich, London. UTC is a time standard, defined by atomic clocks and nudged with the occasional leap second to stay within a second of the sun. UTC is the modern, exact successor; GMT is the older everyday name for the same zero point.
One thing that trips people up: the UK is not on GMT all year. It uses GMT (UTC+0) in winter and British Summer Time (UTC+1) from late March to late October. So "London time" and "GMT" only line up for half the year.
How offsets work — and the odd ones
Most zones are a whole number of hours from UTC, which is why a flight or a meeting time usually shifts by a clean hour or two. But not all of them:
Half-hour zones. India runs the entire country on UTC+5:30; so do parts of Australia (UTC+9:30) and Newfoundland (UTC−3:30).
Quarter-hour zones. Nepal is UTC+5:45 and the Chatham Islands are UTC+12:45 — fifteen-minute offsets, the rarest kind.
One country, one zone. China spans five geographic zones but keeps a single clock, UTC+8, nationwide. India does the same.
Add the whole-hour, half-hour and quarter-hour offsets together and there are around 38 distinct ones in use worldwide — more than the tidy 24 the "24 time zones" phrase suggests.
Daylight saving: why the clocks move
Many countries shift their clocks forward an hour in spring and back an hour in autumn to get more evening daylight in the warmer months — "spring forward, fall back." While it's in effect, that region's offset changes: New York is UTC−5 in winter but UTC−4 in summer, and most of Europe goes from UTC+1 to UTC+2.
This is the real reason "what time is it in London?" has no fixed answer — it depends on the season. And because countries change clocks on different weekends (Europe and the US don't switch on the same date), the gap between two cities can briefly be an hour off from usual for a week or two each spring and autumn. Plenty of places — Japan, India, most of Africa, and Arizona within the US — skip daylight saving entirely and keep one offset all year.
The International Date Line
If every zone east of Greenwich is a bit ahead and every zone west is a bit behind, they have to meet somewhere on the far side of the world — and they do, at the International Date Line, running roughly down the 180° meridian in the middle of the Pacific. It's where the calendar date flips:
Cross it heading west (say, flying from Hawaii to Japan) and you skip forward a day.
Cross it heading east (Japan back to Hawaii) and you live the same date twice.
The line isn't straight — it zigzags around island nations so each country stays on a single date. That's how the islands of Kiribati ended up at UTC+14, the earliest clock on Earth, while nearby waters sit at UTC−12: same moment, a whole day apart on the calendar. The first places to see each New Year are right beside the last.
How computers write the offset: the ISO way
To avoid all this ambiguity, machines attach the offset right to the timestamp using the ISO 8601 standard — the same standard behind year-first dates and ISO week numbers. A time ending in Z means UTC exactly: 2026-06-09T14:30:00Z. A time ending in an offset, like 2026-06-09T16:30:00+02:00, says "this is two hours ahead of UTC" — and it points to the very same instant. Storing the offset with the time is how apps show everyone the right local moment no matter where they open it.
Working across zones
Two habits prevent almost every time-zone mix-up: name the zone whenever you write a time for someone elsewhere ("3 PM your time" is a guess; "3 PM London / 10 AM New York" is a fact), and lean on UTC as the neutral anchor for anything global. Once you've pinned a moment down, the week.hako.to tools handle the date math around it — days between two dates, add or subtract days, the days-until countdown, or turning any date into its ISO week number — all in your browser, nothing sent anywhere.
Offsets here are standard (non-daylight-saving) values and reflect common practice; rules change from time to time and a few regions differ locally. This page is fixed reference text — nothing you read here is sent anywhere.