← All blog posts · 4 November 2026

How Time Zones Work — A Simple Guide

When it is noon in London, it is already 5:00 pm in Karachi and only 7:00 am in New York. Nobody finds that strange — but why does the world run on dozens of different local times at all, why are some of them offset by 30 or 45 minutes instead of whole hours, and how does your phone always land on the right one? Here is the whole system, explained simply.

Why time zones exist

The Earth rotates once every 24 hours, so the sun reaches its highest point — solar noon — at a different moment for every longitude. Before railways and telegraphs, every town simply kept its own local solar time, and that worked fine. It stopped working when trains needed timetables: a journey's departure and arrival towns might disagree about the time by twenty minutes. Countries standardised time within their borders during the 1800s, and in 1884 an international conference chose the meridian through Greenwich, London, as the zero point from which all other times would be measured.

The basic idea is tidy: divide the planet into 24 slices of 15 degrees of longitude, each one hour apart, so that clocks everywhere stay roughly in step with the sun. Real life then untidied it — borders, politics and geography bend the lines, some countries span several zones and others, like China, choose to run one single time across a width that could hold five.

UTC offsets: how every zone is defined

Every modern time zone is defined as an offset from UTC (Coordinated Universal Time): Karachi is UTC+5, meaning its clocks run five hours ahead of UTC; New York is UTC−5 in winter, five hours behind. The difference between any two cities is simply the difference between their offsets — which is exactly what our time difference calculator computes, live, for any pair of cities. The UTC vs GMT guide explains what UTC itself is and how it differs from the older GMT.

Many regions also move their offset twice a year for daylight saving time — forward an hour in spring, back in autumn — which is why the difference between two cities is not fixed. Karachi to New York, for example, is 9 hours in summer and 10 in winter.

Why some offsets are :30 or :45

Whole hours are a convention, not a law of nature. When a country's population sits between two hourly meridians, a half-hour offset can fit solar time better. India uses UTC+5:30 for the whole country; Iran uses UTC+3:30; Newfoundland, in Canada, uses UTC−3:30. Nepal goes one step further with UTC+5:45, the world's only 45-minute offset in regular use, based on the longitude of the Gaurishankar mountain — Australia's Chatham Islands use UTC+12:45. These are not errors or quirks of rounding; they are deliberate national choices, and any correct clock has to honour them, which is why this site derives every offset from the real rules instead of assuming whole hours.

The International Date Line

If you keep adding an hour as you travel east, you eventually gain a whole day — so somewhere the calendar has to jump back. That somewhere is the International Date Line, running mostly down the middle of the Pacific Ocean along roughly 180° longitude, zig-zagging to keep island nations on one side or the other. Cross it westward and you skip a day; cross it eastward and you repeat one. It is why Sydney can be celebrating Tuesday morning while Honolulu is still on Monday.

How your phone always knows

Underneath every phone, computer and website clock sits the IANA time zone database — a carefully maintained public record of every zone's offset rules, past and present, including every daylight saving change. When a government announces a rule change, the database is updated and your devices pick it up with their normal software updates. World Clock uses that same database, through your browser's built-in Intl API, to compute every time on this site — nothing is hardcoded, so the clocks stay right when the rules change. Try it on the live dashboard: search any city in the world and compare it side by side with home.