How time zones work

The Earth rotates once every 24 hours, so at any moment it is morning somewhere, midnight somewhere else, and everything in between. To keep clocks meaningful, the world is divided into time zones — regions that agree to share the same clock time. Each zone is defined by its offset from UTC (Coordinated Universal Time), the global reference. Seoul and Tokyo run at UTC+9, London at UTC+0 in winter, New York at UTC−5. Most offsets are whole hours, but not all: India uses UTC+5:30 and Nepal UTC+5:45.

Zone boundaries follow politics as much as geography. China spans five natural time bands but keeps a single national clock, while mainland USA splits into four. That is why this site tracks time by city rather than by country — a city pins down exactly one set of rules. Every clock here is computed from the IANA Time Zone Database, the canonical rulebook maintained since 1986 and shipped inside every modern operating system and browser.

What daylight saving time changes

Many countries shift their clocks forward one hour in summer — daylight saving time (DST) — and back again in winter. The idea is to move an hour of daylight to the evening. The catch is that regions change on different dates: Europe and North America switch weeks apart, the southern hemisphere switches in the opposite direction, and most of Asia and Africa never switch at all. As a result, the gap between two cities can change several times a year. London and Seoul are usually 9 hours apart, but 8 during British Summer Time. Every clock, difference and conversion table on this site applies the current rules automatically, so you never have to remember which side has switched.

How to use this site

The big clock above shows your own local time, detected from your device. Below it, the world grid gives the live time in major cities — cities are clickable, and each city page adds the full picture: time differences to other major cities, the UTC offset, and whether daylight saving applies. To schedule across zones, the time zone converters translate any hour between two cities and show an hour-by-hour table plus the best window for a call, and the meeting planner lines several cities up on one timeline to find a slot inside everyone's working hours. Looking for somewhere specific? The search box above covers every IANA time zone, and the all cities page lists everything by region.

Everything runs in your browser from your device clock — no account, no install, and nothing personal sent anywhere. If your device clock is right, so is this site.