Direct answer
A time zone is an offset from UTC (Coordinated Universal Time). Most of the world uses 24 main zones in one-hour steps, but ~40 zones exist in practice because of half-hour, 45-minute, and daylight-saving variants.
How time zones actually work
Time zones express each region's local clock as an offset from UTC. UTC+0 is the reference; UTC−5 is five hours behind, UTC+9 is nine hours ahead.
The Earth rotates 360° in 24 hours, so each hour of rotation is roughly 15° of longitude. Time zones approximate this by grouping regions that share a common civil time. Political borders, not pure geometry, decide the final boundaries — which is why China spans five geographic zones but uses a single official time.
Software stores time as a UTC instant and renders it in the viewer's local zone. That separation is what lets a single calendar event display as 14:00 in New York and 19:00 in London without storing two different values.
UTC, GMT, and the difference no one explains
UTC is a 24-hour atomic-clock standard with no time zone of its own. GMT is the historical British civil time, identical to UTC+0 in practice but defined astronomically.
- UTC never observes daylight saving. It is the universal reference instant.
- GMT is a time zone used by the UK in winter; in summer London switches to BST (UTC+1).
- Aviation, finance, and IT systems schedule in UTC to avoid ambiguity.
- Zulu time (Z) is military shorthand for UTC.
Daylight saving time, in one paragraph
DST shifts the clock forward one hour in spring and back one hour in autumn so that summer evenings have more daylight. About 70 countries observe some form of it; most of Asia, Africa, and the equatorial belt do not.
The shifts happen on different weekends in different regions. The US and Canada change on the second Sunday in March and first Sunday in November. The EU changes on the last Sunday in March and October. Australia is reversed — its DST begins in October. This three-to-four-week mismatch creates the trickiest scheduling window of the year.
Global examples you can memorize
Anchor a few reference offsets and you can mentally convert most business cities without a tool.
- New York: UTC−5 standard, UTC−4 in DST.
- London: UTC+0 standard, UTC+1 in BST.
- Berlin / Paris: UTC+1 standard, UTC+2 in CEST.
- Dubai: UTC+4, no DST.
- Mumbai: UTC+5:30, no DST.
- Singapore / Beijing / Hong Kong: UTC+8, no DST.
- Tokyo / Seoul: UTC+9, no DST.
- Sydney: UTC+10 standard, UTC+11 in AEDT.
- Los Angeles / San Francisco: UTC−8 standard, UTC−7 in PDT.
Common mistakes professionals make
Most scheduling errors come from five recurring traps. Knowing them is half the fix.
- Assuming a city's offset is fixed (DST shifts it twice a year).
- Treating GMT and UTC as interchangeable across the year.
- Sending invites in a 'floating' time without an explicit zone.
- Forgetting that Australia's DST runs opposite to the Northern Hemisphere.
- Ignoring 30- and 45-minute offsets (India, Iran, Nepal, parts of Australia).
Frequently asked questions
How many time zones are there in the world?
There are 24 standard one-hour offsets, but in practice around 40 distinct zones exist once half-hour and 45-minute regions and daylight-saving variants are included.
Is UTC the same as GMT?
They share the same offset (UTC+0 = GMT) but are defined differently. UTC is an atomic-clock standard; GMT is a historical civil time. London uses BST (UTC+1) in summer, so even the UK is not on GMT year-round.
Why does India use UTC+5:30 instead of a whole hour?
India adopted a single 30-minute-offset zone in 1955 to balance the difference between its eastern and western extremes under one national clock.
Which countries do not observe daylight saving time?
Most of Asia, Africa, and South America skip DST. Notable opt-outs include Japan, China, India, the UAE, Saudi Arabia, Singapore, and most of Mexico.