World Clock

UTC (Coordinated Universal Time) is the primary international time standard. GMT (Greenwich Mean Time) refers to the time at the Royal Observatory in Greenwich, London, at 0° longitude. For everyday purposes they are interchangeable — both represent UTC+0. The technical difference is that UTC is based on atomic clocks while GMT is an astronomical standard. Most of the world's time zones are expressed as UTC+ or UTC− a certain number of hours.

DST is the practice of advancing clocks by one hour during warmer months so that evenings have more daylight. The United States "springs forward" on the second Sunday in March and "falls back" on the first Sunday in November. Europe changes on the last Sunday of March and October. Many countries do not observe DST at all, including China, Japan, India, Russia (since 2014), and most of Africa. This world clock automatically reflects the current DST status of each city.

Most time zones use whole-hour offsets, but some countries chose non-standard offsets for geographic, political, or historical reasons. India (UTC+5:30) and Sri Lanka (UTC+5:30) use 30-minute offsets. Nepal uses UTC+5:45 — the only 45-minute offset in the world. Afghanistan uses UTC+4:30. Australia's Lord Howe Island uses UTC+10:30. Iran uses UTC+3:30. These offsets were often chosen to place the midday sun as close as possible to solar noon for the country's geographic center.

The International Date Line (IDL) is an imaginary line at roughly 180° longitude in the Pacific Ocean where the calendar date changes. Crossing westward advances the date by one day; crossing eastward moves it back. This is why Samoa and Tonga are nearly a full day ahead of Hawaii despite being geographically close. The IDL is not perfectly straight — it zigzags around island nations so that neighboring communities share the same calendar date.

A UTC offset is simply the number of hours (and sometimes minutes) a location is ahead of or behind UTC. A time zone is a named region that follows the same offset, often with DST rules. For example, "Eastern Time" is a named time zone. In winter it observes EST (UTC−5) and in summer EDT (UTC−4). UTC offsets are static; time zones can change offset twice per year when DST starts and ends. This clock uses IANA time zone identifiers (like "America/New_York") to correctly handle all DST transitions.

Kiritimati (Christmas Island) in Kiribati is the furthest ahead at UTC+14 — it was one of the first places to enter the new year. Among major cities, Auckland, New Zealand (UTC+12 or UTC+13 with DST) and Tokyo, Japan (UTC+9, no DST) are among the furthest ahead. On the other end, the Howland and Baker Islands (uninhabited US territories) use UTC−12, making them one of the last places on Earth to see each new day begin.

This clock uses the browser's built-in Intl (Internationalization) API with IANA time zone database entries to display the accurate local time in any city. Your browser already knows the rules for every time zone, including all historical and future DST transitions. The local time display uses your device's system clock, which should always be accurate if your device is connected to the internet (NTP synchronization). The clocks update every second using JavaScript's setInterval function.

The "golden hours" for international meetings are times when working hours (9am–5pm) overlap across all participants. New York to London: 9am–12pm Eastern (2pm–5pm London) works well. New York to Singapore is very difficult: Singapore's business hours (9am–5pm, UTC+8) correspond to 8pm–4am Eastern. For truly global meetings spanning the Americas, Europe, and Asia, many teams accept that someone will always be on a difficult time. Tools like meeting planners can help find the least-bad overlap window.

China spans roughly five natural geographic time zones (UTC+5 to UTC+9) but officially uses only one: China Standard Time at UTC+8. This was mandated in 1949 for national unity and simplicity. In practice, Xinjiang in the far west informally uses "Xinjiang Time" (UTC+6) for local schedules. The consequence is that sunrise in Kashgar, Xinjiang can be as late as 10am in winter by official clock time. By contrast, Russia uses 11 time zones, and the United States uses 6 for the contiguous states plus territories.

The contiguous US uses four main time zones: Eastern (UTC−5/−4 DST), Central (UTC−6/−5), Mountain (UTC−7/−6), and Pacific (UTC−8/−7). Alaska uses Alaska Time (UTC−9/−8) and Hawaii uses Hawaii-Aleutian Standard Time (UTC−10, no DST for Hawaii). Arizona (except the Navajo Nation) does not observe DST, keeping UTC−7 year-round. US territories add further zones: Puerto Rico (UTC−4), Guam (UTC+10), and American Samoa (UTC−11).