How do time zones work?
Every zone is UTC plus an offset, every offset is a political decision, and every device resolves them through the same shared database.
The short answer
One instant exists; each place labels it differently. Every zone is defined as UTC plus or minus an offset, so converting between any two places is arithmetic on a shared baseline.
The offsets themselves are set by governments, not by geography — which is why zone boundaries follow borders rather than meridians.
The three layers
The baseline. UTC, maintained by atomic clocks, the same everywhere. See what is UTC time.
The offset. Each region's legal difference from UTC — mostly whole hours, sometimes half or quarter hours. Set by law and changeable.
The rules. Whether that offset changes seasonally, and on which dates. Also set by law, also changeable, and different in every region that has them.
Together these mean local time is a function of an instant and a place, not a property of a clock.
Why zones follow borders, not meridians
In a purely geometric world, zones would be 15°-wide bands and boundaries would be straight north–south lines.
Actual boundaries wander enormously, because no country wants an internal time boundary through a city, and neighbours want to share business hours. The result:
- China uses one zone across roughly five geographic hours of longitude.
- Spain sits at the longitude of the UK but uses Central European Time, a decision from the 1940s that was never reversed.
- India uses a single half-hour offset for a country wide enough for two zones.
- The US–Canada boundary jogs repeatedly to keep towns whole.
Time zones are an administrative convenience, and they look like it.
How your device resolves local time
- It knows its zone identifier —
America/New_York,Asia/Kolkata. - It has a copy of the tz database with that zone's full rule history.
- To display a time, it takes the current instant, looks up which offset applied in that zone at that instant, and adds it.
That last step is why the same device shows the right time for a date next summer and for a date in 1998 — it applies the rules that were in force then, not the ones in force now.
Why it goes wrong
Almost every time zone bug comes from storing the wrong thing:
- Storing a fixed offset rather than a zone: frozen at one season.
- Storing text like "3pm EST": nothing converts it.
- Doing arithmetic on local times: adding 24 hours across a transition gives the wrong answer, because the real elapsed time is 23 or 25 hours.
The general fix is the same everywhere: store instants in UTC, store places as zone identifiers, and convert only for display.
Where it came from
Local solar time worked fine until railways, at which point a timetable needed both ends to agree what time it was. Britain adopted railway time in the 1840s, the US adopted Standard Railway Time in 1883, and the 1884 International Meridian Conference set Greenwich as the reference.
So the whole system is about 140 years old and was invented to stop trains crashing.
Related: Why do time zones exist? · The tz database
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.