How to Use

1. The clock displays 6 major cities by default (New York, London, Tokyo, Sydney, Dubai, Los Angeles).
2. Click the Add a timezone dropdown to add more cities to your display.
3. Click the remove button on any clock card to remove it from the display.
4. Toggle between 12-hour and 24-hour format with the format button above the clocks.

Frequently Asked Questions

Why is the time different from my expected timezone?

The clock uses your browser's local timezone database. The time shown is accurate based on the selected city's timezone and any applicable daylight saving time rules.

Can I save my favorite cities?

Your selections are saved in your browser's localStorage, so they will persist when you revisit the page. This data never leaves your device.

How often does the clock update?

The clock updates every second to show the live current time. All cities update simultaneously to maintain accuracy across timezones.

A short history of standard time

Until the late 1800s every town set its clocks to local solar noon, the moment the sun crossed the meridian directly overhead. Two villages 20 km apart could legitimately disagree by a minute, which was harmless when people travelled by horse but broke down completely once printed railway timetables existed. Britain bit the bullet first: the Railway Clearing House standardised Greenwich Mean Time (GMT) across the network in 1847, nicknamed "railway time."

The 24-hour world-zone idea is usually credited to the Canadian railway engineer Sandford Fleming, who proposed it in 1879 after famously missing a train in Ireland because a printed schedule listed "p.m." instead of "a.m." Five years later the International Meridian Conference in Washington, DC settled the political question. On 13 October 1884, twenty-six countries voted 22 to 1 with 2 abstentions (the Dominican Republic against; France and Brazil abstaining) to adopt the meridian through Greenwich as the world's prime meridian. France, characteristically, sulked: until 1911 Paris officially referred to its time as "Paris mean time, retarded by 9 minutes and 21 seconds" rather than say the word "Greenwich."

GMT served as the world's reference for most of the 20th century. Coordinated Universal Time (UTC) superseded it as the international civil standard on 1 January 1972, built on the weighted average of hundreds of atomic clocks tracked by the BIPM. UTC and GMT can differ by up to 0.9 seconds; the difference is reconciled with leap seconds, which the General Conference on Weights and Measures voted in 2022 to abolish by 2035.

The IANA tz database

Every serious world clock, including this one, relies on the IANA Time Zone Database, also known as the Olson database after its founder Arthur David Olson, who began it at the US National Institutes of Health in the mid-1980s. ICANN took institutional responsibility in October 2011, and Paul Eggert has been editor since 2005. New releases ship several times per year, whenever a country changes its DST rules, redraws a zone, or some historical record is corrected. Both the data and the reference C source are in the public domain.

Zone identifiers follow the format Area/Location, for example America/New_York, Europe/London, Asia/Tokyo. The "Location" is a representative city rather than a country, because political borders move and time-rule histories follow geography. Modern programming environments (JavaScript, Python's zoneinfo, Java, .NET, PostgreSQL) all accept and prefer IANA names rather than abbreviations.

Why no abbreviations? Because they collide:

• EST can mean US Eastern Standard Time (UTC−5) or Australian Eastern Standard Time (UTC+10).
• CST can mean US Central (UTC−6), China Standard (UTC+8), or Cuba Standard (UTC−5).
• IST can mean India (UTC+5:30), Irish Summer (UTC+1), or Israel Standard (UTC+2).
• BST can mean British Summer Time (UTC+1) or Bangladesh Standard Time (UTC+6).

Asia/Kolkata is unambiguous in a way "IST" never can be.

Daylight saving time, briefly

Benjamin Franklin's 1784 letter to the Journal de Paris suggesting Parisians wake earlier to save candles is the most-misattributed item in the entire DST corpus: it was a satire and did not propose changing the clocks. The first modern proposal was by the New Zealand entomologist George Hudson in 1895; the first national adoption was Germany and Austria-Hungary on 30 April 1916, as a wartime measure to conserve coal. The US followed in 1918 under the Standard Time Act, also as a wartime measure.

Today the rules vary widely:

• United States. DST runs from the second Sunday in March to the first Sunday in November, an extension of the older schedule that the candy industry famously lobbied for so trick-or-treating would happen in daylight. Hawaii (since 1967) and most of Arizona skip it. The Navajo Nation within Arizona does observe DST so it stays aligned with members in Utah and New Mexico, so Tuba City and the neighbouring town of Moenkopi are about two miles apart and one hour out of step in summer.
• Most of Asia, Africa, and equatorial Latin America never observed it. Brazil abolished DST in 2019. Mexico abolished it in late 2022, though some northern border municipalities still observe it to align with Texas and California. Russia abolished it in 2011.
• European Union. Parliament voted in March 2019 to end seasonal clock changes and let member states pick permanent standard or permanent summer time, but the Council has never confirmed. As of early 2026 the proposal remains stalled, with no agreed end date, primarily because neighbouring countries can't agree which way to standardise. The current rule (forward last Sunday in March, back last Sunday in October) remains in force.

Health and economic effects are modest at best: a 2017 study estimated DST's spring transition causes around 30 deaths per year in the US through sleep deprivation, and meta-analyses put the electricity savings at roughly 0.3% on the days it applies. The American Academy of Sleep Medicine has formally recommended permanent standard time as the healthiest option.

The exotic offsets

The world is not on a clean grid of integer hours. Real political geography is messier:

Offset	Where	Note
UTC+14:00	Kiribati (Line Islands)	Easternmost time on Earth. Was UTC−10 until 1995 when Kiribati moved the date line east.
UTC+13:00	Samoa, Tonga	Samoa famously skipped 30 December 2011 entirely to switch from UTC−11 to UTC+13.
UTC+12:45	Chatham Islands (NZ)	One of the world's only :45 offsets.
UTC+10:30	Lord Howe Island (AU)	The world's only 30-minute DST shift (to UTC+11).
UTC+5:45	Nepal	Set as 5°45′ east of Indian Standard Time, the only :45 offset on a sovereign mainland.
UTC+5:30	India, Sri Lanka	A single zone covers a country wider than the continental US.
UTC−3:30	Newfoundland, Canada	Famous half-hour zone.
UTC−12:00	Baker & Howland Islands	Westernmost time. The date can differ by 26 hours from UTC+14.

The International Date Line is not a straight line. It zigzags east around Kiribati so the country is on one side, and west around the Aleutians so all of Alaska shares a date with the rest of the US.

How a browser-based world clock works

Every JavaScript Date object internally stores a single number: milliseconds since 1 January 1970 00:00 UTC, the Unix epoch. It's timezone-agnostic. The same instant looks identical in Tokyo and in New York. The browser exposes the IANA tz database via Intl.DateTimeFormat with a timeZone option:

new Intl.DateTimeFormat("en-US", {
  hour: "numeric", minute: "numeric", second: "numeric",
  timeZone: "Asia/Tokyo", hour12: false
}).format(new Date());
// "23:14:07" if it's 23:14 in Tokyo right now

This is exactly the operation each card performs once per second. Intl.DateTimeFormat with the timeZone option has been broadly available across Chrome, Firefox, Safari and Edge since around 2017. The browser ships whichever tz database version your OS has installed; old, unpatched mobile devices may serve stale rules, which is one reason this tool re-fetches the current rule set on every page load rather than baking it into the JavaScript.

When you'd reach for one

• Distributed teams. All-remote companies like GitLab, Automattic and Zapier explicitly recommend a world-clock tool in their internal handbooks. Open-source projects with global contributors do the same.
• Scheduling international meetings. A sales rep in London setting up a 4 p.m. local call with Tokyo needs to confirm that's midnight in Tokyo before pressing send.
• Stock and currency markets. Trading is calendared by each exchange's local time. NYSE 09:30 to 16:00 in New York is 14:30 to 21:00 UTC during EST. The London/New York overlap (roughly 14:30 to 16:30 UTC) is the highest-liquidity window of the global trading day. Forex itself runs continuously from the Sydney open Sunday 22:00 UTC to the New York close Friday 21:00 UTC.
• Sport and broadcasting. A Champions League final's local kick-off in Madrid is one piece of information; what time it airs in Sydney, Mumbai and São Paulo is the rest of the broadcasting rights schedule.
• Family and personal: calling home. A grandparent in Toronto and a grandchild on Erasmus in Lisbon: set both side by side once and never wonder again whether 8 a.m. is too early to call.
• Aviation and shipping. International pilots schedule by UTC ("Zulu" time) to avoid ambiguity, but their human bodies live on local rest cycles. Crew apps run UTC and local side by side.

More questions

Why does my clock keep showing the right time even when DST starts?

Because the IANA tz database knows the rules for every zone: when DST starts and ends each year, when those rules changed historically, and when individual countries abandoned them. The browser applies the right rule automatically based on the current instant. If your operating system is up to date, recent changes (Mexico abandoning DST in 2022, the EU's still-open question) are already reflected.

Is GMT the same as UTC?

In casual British use yes; technically no. GMT is the older astronomical reference based on Earth's rotation; UTC is the atomic-clock-based modern civil standard, in force since 1 January 1972. They can differ by up to 0.9 seconds, which the leap-second mechanism reconciles. The 2022 General Conference on Weights and Measures voted to abolish leap seconds by 2035, after which UTC will be allowed to drift slightly from astronomical UT1 over long timescales.

Why does the date sometimes look different in the next column?

Because the world is more than 24 hours wide once you include UTC+14 (Kiribati) and UTC−12 (US uninhabited islands). The date can differ by 26 hours between the easternmost and westernmost zones, meaning that for two hours every day, three calendar dates exist on Earth at the same instant.

Where do my saved cities go?

Into your browser's localStorage, which is a small private store that lives on your device, not a server. Clearing your browser data clears them. Switching browsers or devices means starting again, because nothing was synced anywhere.

Does anything get sent to a server?

No. The clock reads the current time from your local machine, formats it for each chosen zone via the browser's built-in internationalisation API, and updates every second using a JavaScript timer. The page works offline once it's loaded.

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