How this world clock works

A world clock looks like the simplest tool on this site and is quietly one of the fussier ones. The naive version reads your device's clock once, adds a fixed number of hours per city, and updates every second. Each of those three choices is wrong in a way you only notice at the worst moment.

Offsets are recomputed, never added

An offset is not a property of a place. It is a property of a place at an instant. London is one hour ahead of UTC in August and level with it in December, and the switch happens at a moment fixed by law rather than by arithmetic. So this page never stores "London is +1". It asks, for the exact instant now being displayed, what London's clock reads — and does that again on every update.

The practical consequence is that the page cannot be wrong for an hour twice a year, which is the classic world-clock failure. It also means a city added in June still reads correctly in November without anything being refreshed.

Nothing counts up

Every reading is recomputed from your device's clock rather than incremented from the previous one. That distinction matters because browsers deliberately slow down timers in background tabs — Chrome to once a minute, Firefox on Android to once every fifteen minutes, and both may discard the page entirely. A clock that counts up loses exactly the ticks it was denied and stays wrong until reloaded. A clock that recomputes is briefly stale and then correct again, which is a very different kind of failure.

One timer for every city, not one each

Every city on your list — up to twenty-five — shares a single timer. It re-derives its own alignment from the clock on every tick, so it cannot drift out of phase the way a repeating interval does, and it wakes the device far less often than one timer per city would. On a phone that is battery you keep.

Why the default is minutes, not seconds

Out of the box these clocks update once a minute, on the minute. That is a deliberate default and not a limitation — you can turn seconds on whenever you want them.

Updating once a minute wakes your device sixty times less often, and the display is never more than a moment out of date because the tick is aligned to the minute boundary rather than to whenever the page happened to load. It also matters for anyone using a screen reader: a page that changes sixty times a minute can make the surrounding content genuinely hard to navigate.

That is also why there is a Pause clocks button. Accessibility guidelines require that information which starts updating on its own can be paused, stopped, or slowed — and this page starts the moment it loads, so the requirement applies squarely. Rather than treat that as a box to tick, both controls are ones we think you will actually use: pausing is the easiest way to screenshot a set of times, and seconds are genuinely useful when you are counting down to a market open or a launch.

Reading the extras

Yesterday and tomorrow

Each city shows how its date relates to yours. This compares actual calendar dates rather than the difference between offsets, because those two are not the same question. Tokyo is eight hours ahead of London through British Summer Time and nine in winter, and shares its date for most of the day; Berlin is one hour ahead all year and lands on a different date for one hour out of every twenty-four.

Where a city is two calendar days away from yours — which is possible, because the world spans twenty-six hours of legal offsets rather than twenty-four — the page shows the date and no word. There is no honest one-word way to say "two days ahead", and stretching "tomorrow" to cover it would be a confident wrong answer.

The next clock change

Each city shows when its clocks next move, found by searching that zone's actual rules rather than by looking anything up in a table we maintain. That matters for planning: a recurring call that works today can land an hour differently for one participant after their country changes its clocks and yours does not, and the gap between the two dates is where the mistake lives.

The amount is read from the rule rather than assumed. Most places move by an hour; Lord Howe Island moves by thirty minutes and Antarctica's Troll station by two, so the sentence says what that particular place actually does. A city that does not observe daylight saving simply shows nothing.

Worked examples

A recurring call that survives October

A team in London, New York and Sydney books a weekly call at 09:00 London time. In late October the UK moves its clocks and the US follows a week later, while Australia has already moved in the opposite direction. For that one week the call is an hour off for New York relative to where everyone expects it. Adding the three cities here shows each one's next change date, so the gap is visible before it bites rather than after.

A market open

Watching an exchange open from another country is the case where seconds earn their keep. Turn seconds on, add the city, and the readout is precise to the second and aligned to your device's clock — with no assumption that your offset and theirs move together.

What this page cannot tell you

Time zone rules come from your own device. Every modern browser ships a copy of the IANA Time Zone Database and exposes it through a standard interface, which is the same data operating systems and airlines use — but it is your copy, updated by your operating system.

If a country changes its clocks and your device has not yet received the update, this page will show that country's old rules and has no way to know. This page makes no claim about whether your device's data is current, because the only honest way to make that claim is to maintain a list of recent changes and check it constantly — and a list like that quietly stops being accurate the moment nobody updates it, while still reporting that everything is fine. We would rather say nothing than offer a reassurance we cannot keep. Keeping your operating system up to date is what actually protects you here, and for anything consequential, check against an official source.

Place and country names come verbatim from Unicode CLDR and the IANA Time Zone Database, and this site takes no position on any territorial claim. A territory appears in the list because IANA's country file carries the code — inclusion is a fact about data completeness, not recognition. There are no maps, no flags and no borders anywhere on this site, deliberately and permanently. We quote the time zone database's own header on the point: “It is not intended to take or endorse any position on legal or territorial claims.” The full policy is on the about page.

Everything on this page is saved in your browser only. Clearing site data or switching devices will erase your list.

Questions

Do my cities sync between devices?

No. There is no account and nothing leaves your browser, so a list built on your laptop will not appear on your phone. That is the trade for a tool that needs no sign-up and holds nothing about you.

Why does my city not appear in the list?

The picker lists one entry per time zone rather than one per city, labelled with the city the zone is named after. If yours is missing, pick any place that keeps the same clocks — the times shown will be identical, because they are the same zone.

Can I rename a city?

Not yet. Rows are identified by their time zone rather than their label, which is what stops a list saved in one browser turning into duplicates in another, so renaming is a change we want to get right rather than ship quickly.

Why is the clock not moving?

Check whether the Pause clocks button is on — it stays paused across reloads, because a pause that quietly undid itself would be worse than no pause at all.

Is 25 cities really the limit?

Yes. Beyond that the page stops being something you can read at a glance, which is the only thing it is for. Adding a twenty-sixth removes the oldest and tells you it did.