UTC: the world's reference time
Coordinated Universal Time (UTC) is the time standard every clock is set by. It is kept by atomic clocks around the world and does not change with the seasons. Every time zone is described as an offset from it: New York in winter is UTC−5, Tokyo is UTC+9.
GMT (Greenwich Mean Time) is often used as if it meant UTC. Strictly, GMT is the time zone the UK uses in winter; for everyday purposes it is the same as UTC. UTC is occasionally adjusted by a "leap second" to stay in step with the Earth's rotation, but in 2022 the international body that oversees it decided to stop adding them by 2035.
How many time zones are there?
In theory the world is 24 one-hour slices, each 15° of longitude wide. In practice the borders follow countries, not lines on a map. The official IANA time zone database lists 418 zones, and at some point in 2026 they use 39 different offsets, from UTC−11 to UTC+14.
13 of those offsets are not whole hours:
- UTC−9:30 — the Marquesas Islands (French Polynesia)
- UTC−3:30 — Canada
- UTC−2:30 — Canada
- UTC+3:30 — Iran
- UTC+4:30 — Afghanistan
- UTC+5:30 — Sri Lanka and India
- UTC+5:45 — Nepal
- UTC+6:30 — Cocos Islands and Myanmar
- UTC+8:45 — Eucla (Australia)
- UTC+9:30 — Australia
- UTC+10:30 — Australia
- UTC+12:45 — the Chatham Islands (New Zealand)
- UTC+13:45 — the Chatham Islands (New Zealand)
Some countries are much wider than one zone but keep a single time: all of China uses UTC+8, although the country spans about five zones' worth of longitude, so in the far west the sun can rise after 9 am in winter.
The International Date Line
Opposite Greenwich, roughly along the 180° meridian in the Pacific, is the date line: cross it going west and you skip a day; going east you repeat one. It zigzags around island nations so that each keeps one date. Samoa moved from one side to the other in 2011, skipping 30 December entirely, and part of Kiribati is so far east of the line's old position that it is on UTC+14 — the first place in the world to see each new day.