World Clock

How do you compare times with our world clock?

A good world clock helps you notice the date, not just the digits. Add the places you care about, choose a clear reference, and switch to planning when a conversation needs one exact start.

1. Add the places that matter

   Search for a city and add it from the suggestions. We keep each place once, show its country and time zone, and let you move your main city first.

2. Choose live time or a meeting

   Live follows your device clock. Planning asks for a date, local time and reference zone, then holds that instant still. A missing or repeated local time needs your attention before applying it.

3. Check the date, then share

   Read every city’s date as well as its hour. The overlap chart helps you compare chosen workdays. Copy a timestamped summary or share the setup; a planned link preserves the same instant.

A better time to connect

Make the comparison your own

How did we start sharing time across the world?

Our world clock makes a shared moment look simple, but that simplicity rests on centuries of observation, engineering and agreement. Here is the longer story of how people moved from a town’s own solar noon to a reference that connects the world.

1. Before a shared clock, every town had its own noon

   24 h

   Imagine arranging a journey when the clock in the next town does not quite agree with yours. Before standardized civil time, local solar time gave communities a natural reference: noon belonged to the Sun’s passage across their own meridian. Because Earth turns, that passage happens earlier farther east and later farther west. Neither town’s clock had to be faulty. They could simply be answering the question ‘What time is it here?’ using different geographical starting points.

   In the idealized mean-solar picture, a full rotation is divided into 24 hours: 15 degrees of longitude correspond to an hour, and one degree to four minutes. That is a useful explanation of why longitude and time are connected, not a formula for today’s legal clock time. A sundial follows the apparent Sun, while mean solar time smooths out its uneven pace through the year. Our modern clocks add another layer: a community follows an agreed civil-time rule, even when its clock’s twelve o’clock does not coincide with local solar noon.

   Royal Museums Greenwich

2. Railways made the differences inconvenient; telegraphs made sharing practical

   1840–1852

   Local time was easier to live with when journeys were slow and most appointments stayed within one town. Railways changed the scale of coordination. A timetable had to connect departure, arrival and onward travel across a network, not merely describe the clock at one station. A few minutes’ disagreement could turn a simple instruction into an awkward calculation. British railway companies increasingly adopted Greenwich time during the 1840s, giving their schedules a common reference while local customs continued to change at different speeds.

   Agreement was only half the problem. People also needed a way to learn the reference time and keep their clocks close to it. In 1852, the Shepherd master clock at the Royal Observatory, Greenwich, began distributing time signals by telegraph. An electrical signal could carry a timing reference across a distance far faster than a person carrying a watch. This is the important change: timekeeping became a service that connected places. A station, office or public clock could follow a shared signal rather than independently determine its own noon.

   Royal Museums Greenwich · 1852

3. Greenwich became a common origin, not a switch that synchronized every country

   1884

   A world that exchanged maps, navigation information and schedules needed a shared geographical starting line. The International Meridian Conference of 1884 selected Greenwich as the common prime meridian. That decision gave longitude a widely shared zero and helped international timekeeping work from a common reference. But it did not instantly produce the familiar map of present-day time zones, and it did not mean that every country’s wall clocks suddenly displayed the same hour.

   It helps to separate three questions: where longitude begins, how an international time reference is maintained, and which clock time a country chooses for everyday life. They are related, but they are not identical. Governments and communities developed civil-time arrangements for transport, administration and local needs. Some countries use one zone; others use several. The modern world clock therefore needs the rules attached to a place and a date. Drawing equal-width bands around Earth or dividing a city’s longitude by fifteen cannot reliably tell you what its legal clocks should show.

   Royal Museums Greenwich

4. The time-zone map tells a human story as well as a geographical one

   IANA

   Time zones are often introduced as neat strips on a classroom globe. Real life is less tidy. Borders, regional decisions and seasonal clock changes shape civil time, while some places use half-hour or quarter-hour offsets rather than whole hours. India and Nepal make a useful comparison: their clocks differ by fifteen minutes, even though both sit within the broad Asian part of a world-time map. These are not rounding errors. They are the clock conventions people actually use.

   The date matters just as much as the hour. At one shared instant, a person may still be finishing Monday while another has already begun Tuesday. A late-evening family call or an early-morning handover can cross that boundary without either participant noticing a change of day. We keep the date beside each clock for that reason. A named IANA zone also carries more meaning than an offset written on its own: it identifies a set of civil-time rules that may change over time, rather than permanently promising one numerical difference from UTC.

   IANA Time Zone Database

5. Atomic clocks changed how we measure a second; UTC gave the world a reference

   1967–1972

   As technology demanded steadier timing, the rotation of Earth was no longer enough to define every detail of an exact second. Atomic timekeeping offered a reproducible physical reference. In 1967, the SI second was defined using the cesium-133 transition frequency: 9,192,631,770 cycles correspond to one second. The significance is not merely that a clock became more accurate. Laboratories could realize the same unit from the same physical definition instead of relying only on astronomical observations or the behavior of a particular mechanical clock.

   Coordinated Universal Time, UTC, connects atomic timekeeping with the world’s civil-time reference. The modern leap-second system began in 1972, allowing adjustments that keep UTC related to Earth’s rotation. A leap second is not the same thing as daylight saving: one concerns an international time scale, while the other changes a region’s civil clock. GMT remains a familiar term for zero-offset time, but UTC is the reference we use when describing a shared instant. Our browser tool converts that instant for your chosen places; it does not operate an atomic clock, distribute an official time signal or certify the accuracy of your device.

   NIST · SI second · NIST · UTC

6. Today we share an instant, then give it the right local date and time

   UTC

   International timekeeping is a collaboration, not simply one master clock sitting somewhere on Earth. Time laboratories compare their clocks, and the BIPM uses contributed clock and time-transfer data to compute the international reference. That common foundation lets systems in different places coordinate events without demanding that everyone live by the same local hour. For an ordinary user, the distinction becomes practical when an invitation has to mean the same meeting in London, Mumbai and Sydney.

   A useful digital approach is to preserve one instant and display it through each place’s time-zone rules. The invitation is one event; its local hour and even its calendar date can differ. Our World Clock follows that idea in Planning: changing the comparison reference changes how you read the event, not when it occurs. We also let you compare with UTC or a named zone without adding another city to the board. The history has brought us from separate local noons to shared references, but the everyday goal is still wonderfully human: help people arrive, call, work and celebrate together at the intended time.

   BIPM · UTC · MDN · Intl.DateTimeFormat

One instant, many local dates

UTC → IANA · Time-zone rules → Local time and date

How are the times calculated?

We read one device-clock instant for all live cards, then format it through Intl.DateTimeFormat with each city’s IANA zone. Dates and offsets are calculated from structured parts, not a parsed display string. Planning uses the Temporal polyfill to resolve a local date and time, including gaps and folds. GeoNames supplies city-to-zone mappings, not a live time feed. The comparison picker combines a bundled IANA country-to-zone directory with the named zones your browser supports, validates each zone and groups recognized aliases. This extends beyond the GeoNames city index. The directory identifies places; the browser’s time-zone data still supplies the actual conversion rules.

The device clock, browser time-zone data and city mappings can be wrong or outdated. Historical and far-future rules are not guaranteed; planning is limited to 2000–2099. Work hints are configurable samples, not calendar availability. Reconfirm important bookings with the organizer or official schedule. No alarm, flight tracking or background meeting notification is provided.

Questions worth asking

Sources and corrections

• IANA Time Zone Database
• Temporal
• MDN: Intl.DateTimeFormat
• GeoNames · CC BY 4.0
• NIST: UTC
• Royal Museums Greenwich
• MDN · Intl.DateTimeFormat
• Royal Museums Greenwich · 1852
• NIST · SI second
• NIST · UTC
• BIPM · UTC
• Report a problem

Clock · Alarm Clock · Timer

Found a wrong city, date or offset? Send the city names, IANA zones, chosen date, reference, browser and expected result. Include a primary source for a rule change when possible. A screenshot without private information helps us reproduce the problem.