GMT Converter

The Greenwich Hub Dial

Common GMT Times

Jump the hub to a frequently-converted Greenwich time.

GMT → City Conversion Table

Need the atomic-standard view instead? Open the UTC Converter.

The GMT Conversion Formula

Spoke Cities & Offsets (IANA tz)

No saved times yet. Tap "Save This GMT Time" to remember up to six.

How to Use the GMT Hub Dial

1. Set the Greenwich Mean Time you want to convert using the time input, the preset chips, or the Use Current GMT Now button.
2. Read the central hub for the GMT value and watch every spoke city update its local time simultaneously around the wheel.
3. Check the sun or moon icon on each spoke to see at a glance which cities are in daylight and which are in night for that GMT time.
4. To convert a local time back to GMT, subtract the city's offset from the formula card, or use the Quick Conversion strip for GMT and IST.
5. Save the GMT time to history, and add one hour to any city currently on daylight saving before you rely on the result.

Greenwich Mean Time: The World's Anchor

In 2026, a yacht-delivery skipper logging a transatlantic passage, an air-traffic dispatcher filing a flight plan, and a remote developer joining a London standup all reach for the same anchor: Greenwich Mean Time. The GMT Converter renders that anchor as a hub dial — GMT sits at the centre, and spokes radiate out to a dozen world cities, each showing its own local clock for whatever GMT time you set. Turn the central value and the whole wheel updates at once, which is faster to read than a column of plus-and-minus offsets.

Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London, the prime meridian of 0 degrees longitude established by the 1884 International Meridian Conference in Washington. Before radio time signals, ships set their chronometers to GMT at Greenwich and carried it to sea to compute longitude — the practical reason the prime meridian runs through that particular hill. The Shepherd Gate Clock at the Observatory, installed in 1852, was the first to show GMT publicly to the world.

GMT was the world's civil time reference until 1972, when it was superseded by Coordinated Universal Time (UTC), an atomic standard kept within 0.9 seconds of GMT by periodic leap seconds and coordinated by the Bureau International de l'Heure and now the International Bureau of Weights and Measures. For everyday conversion the two are interchangeable: London sits at GMT and UTC+0 in winter, so a GMT time set on the hub dial is also a UTC time. Aviation, shipping, and the military still say Zulu time for UTC, a direct descendant of the GMT tradition.

The spoke offsets come from the IANA Time Zone Database (the tz database, zoneinfo, or Olson database after founder Arthur David Olson), the canonical source used by Linux, macOS, Java, Python, and the browser's Intl.DateTimeFormat API. Each spoke is labelled with an Area/Location identifier such as America/New_York, Asia/Kolkata, and Australia/Sydney. Setting GMT to 12:00 lights up New York at 7:00 AM (GMT-5), Mumbai at 5:30 PM (GMT+5:30), Tokyo at 9:00 PM (GMT+9), and Auckland at midnight (GMT+12) — the full sweep of the working world in one glance.

India Standard Time is the spoke that proves the dial handles the awkward cases. At GMT+5:30 it is one of only a handful of half-hour offsets, alongside Iran (GMT+3:30) and parts of Australia (GMT+9:30), and was standardised in 1906 around the 82.5 degrees east meridian near Mirzapur. On the hub dial Mumbai's spoke always lands on the :30, so a 9:00 AM GMT meeting reads 2:30 PM in Mumbai, never 2:00. The clean radial layout makes the half hour obvious instead of buried in a table.

The hub-and-spoke metaphor is older than software. Nineteenth-century railway and telegraph offices kept a master Greenwich clock with secondary dials for distant stations, and the world-clock walls of newsrooms, trading floors, and hotel lobbies are the same idea rendered in brass. This converter modernises it: the central GMT value is the master, the spokes are the secondaries, and because the calculation is pure offset arithmetic against the IANA database, the dial agrees with any calendar app, aviation flight strip, or maritime log that also anchors on GMT/UTC.

Daylight Saving Time is the one honest caveat the dial flags rather than fully simulates. The spoke offsets are standard-time values, so during DST a city shifts an hour: the United States moves on the second Sunday of March and first Sunday of November under the Energy Policy Act of 2005, the European Union on the last Sundays of March and October under EU Directive 2000/84/EC, and India, Japan, and most equatorial regions never change. Greenwich itself observes British Summer Time (GMT+1) from late March to late October, which is why pedants distinguish GMT from London local time in summer. Always confirm a booked time against a live, DST-aware calendar before relying on it.