Before clocks were standardized, most towns kept their own local time based on when the sun was directly overhead. That worked fine when travel was slow, but it became unworkable once railways let people move faster than the sun's apparent position changed, and a single train timetable had to make sense in dozens of towns at once.
Time zones are the solution: broad bands, roughly following meridians of longitude, that share a single clock time and a single offset from UTC. Today there are around 40 distinct UTC offsets in active use, from UTC-12 to UTC+14.
From railway time to a world map
Railway companies in the 19th century were among the first to force local towns onto a shared schedule, since a train could not run on a timetable that meant something different in every station it passed through. The idea generalized into standard time zones, and an international conference in 1884 fixed the Greenwich meridian as the reference line from which zones would be measured, giving the world a common zero point.
In principle, each zone spans 15 degrees of longitude, matching the 24 hours it takes the earth to rotate once. In practice, zone boundaries follow national and regional borders far more than lines of longitude, so a zone's shape on a map is usually political, not geometric.
Why some offsets are not whole hours
Most zones sit at a whole-hour offset from UTC, but several countries chose fractional offsets to better match the sun's position across their territory or to align partially with a neighbor. India uses UTC+05:30 nationwide. Nepal uses UTC+05:45, one of only a handful of 45-minute offsets in the world. A number of Pacific and Australian zones use 30-minute offsets as well.
| Offset | Example location |
|---|---|
| UTC-12:00 | Baker Island (uninhabited, far west Pacific) |
| UTC+05:30 | India |
| UTC+05:45 | Nepal |
| UTC+14:00 | Kiribati (Line Islands) |
How software keeps track of it all
A time zone is more than a single offset, because many zones also observe daylight saving time and because the rules governing offsets and transitions have changed repeatedly over the decades, sometimes for political reasons. To handle this, most operating systems and programming languages rely on the IANA time zone database, a public, continuously maintained collection of rules for every observed zone, identified by names such as Europe/London or Asia/Kolkata rather than by abbreviation alone.
When you look up a city's current time, the calculation is really: take UTC, find the applicable rule for that city's zone on that specific date, and apply the resulting offset. That is also how our world clock resolves every city you search for.
Two zones, one confusing overlap
Because daylight saving time is not observed everywhere and does not switch on the same dates everywhere it is observed, the difference in hours between two cities can change several times a year even though neither city has redefined its own time zone. This is a frequent source of scheduling errors for people working across regions, and it is worth double-checking with a live source rather than a fixed rule of thumb.
For a plain-English rundown of how UTC itself is defined and measured, see What Is UTC?, and if you regularly see three-letter codes like CET or IST and are not sure what they refer to, our guide on time zone abbreviations covers the ambiguous ones.
Why the map keeps changing
Countries occasionally adjust their time zone boundaries or drop daylight saving time altogether, usually for economic or political reasons rather than anything astronomical. Because these changes are folded into the IANA database as they happen, software that depends on it stays accurate without needing you to track every change yourself.