World Clock

How time zones work

The world is divided into 24 nominal time zones, each one hour apart, but the real picture is messier. Some countries use offsets that aren't a whole number of hours — India is UTC+5:30, Nepal is UTC+5:45, parts of Australia are UTC+8:45. Russia spans 11 time zones; China, for political convenience, uses just one. Around 70 countries observe daylight saving time, and most don't shift on the same day. The standard time-zone information your phone and computer use keeps track of all of this, and is what powers the times you see above.

GMT and UTC — what they mean

GMT stands for Greenwich Mean Time, the historical reference clock kept at London's Greenwich observatory. Since the 19th century, every other time zone in the world has been defined relative to it. UTC — Coordinated Universal Time — is the modern atomic-clock-based equivalent, introduced in 1972. (The acronym doesn't match the English word order on purpose: it was a compromise between English-speakers who wanted "CUT" and French-speakers who wanted "TUC" — Temps Universel Coordonné — so UTC was settled on as an abbreviation matching neither.) For all practical purposes the two are the same: they're kept aligned within a fraction of a second, and you'll see offsets written either way. "GMT+1" and "UTC+1" mean the same thing.

When you see "GMT-5" under New York, that's the same as saying New York is five hours behind the global reference time. London itself is at GMT (or UTC) in winter, then shifts forward an hour to GMT+1 — also known as BST, British Summer Time — between late March and late October. The reference clock itself never shifts; only the local offsets that countries choose to use.

Why some offsets shift twice a year

You may notice the UTC offset for a city changing twice a year. That's daylight saving time. In the northern hemisphere, most observing countries shift their clocks forward an hour in March and back in October. In the southern hemisphere, the dates are reversed. Our world clock handles this automatically — when London moves between GMT and BST, the offset under the London card updates without you doing anything.

Tips for using the world clock

• Add cities you care about. Use the "+ Add city" button to pick from our list of 65+ major cities. Your selection is saved in your browser, so it'll be there next visit.
• Switch between 12 and 24-hour formats. The toggle next to "Add city" remembers your preference.
• Watch for the day labels. When a city is far enough ahead or behind, you'll see "Tomorrow" or "Yesterday" next to the date — useful for avoiding late-night accidental phone calls.

Who uses a world clock

A live world clock earns its place on a browser tab for anyone whose day touches more than one time zone. Remote workers keep an eye on when colleagues in other offices are actually awake before firing off a message. Families spread across countries check whether it's a reasonable hour to call a parent or a grandchild abroad. Traders and finance staff track when the London, New York, and Tokyo markets open and close. Travellers glance at the destination clock while packing to pre-empt jet lag, and sports fans work out when a match on the far side of the planet actually starts in their own evening. The common thread is a small, constant question — "what time is it there right now?" — that a world clock answers at a glance, without arithmetic.

Time zones that catch people out

Most of the world sits on tidy whole-hour offsets, but the exceptions are the ones that cause missed calls:

• Half-hour and quarter-hour zones. India runs on UTC+5:30, Nepal on UTC+5:45, and parts of Australia on UTC+8:45 or UTC+9:30. Assume a whole-hour offset and you'll be 30 to 45 minutes out.
• One clock for a vast area. China spans what would naturally be five time zones but uses a single one (UTC+8), so the sun rises hours "late" in its far west.
• Places that opt out of daylight saving. Most of Arizona doesn't change its clocks, so its gap from the US East Coast shifts twice a year even though Arizona itself never moves.
• The international date line. Cross it and you gain or lose a whole calendar day — which is exactly why the "Tomorrow" and "Yesterday" labels on the clock cards matter.

How accurate is it?

The world clock reads the current moment from your own device and applies each city's time-zone rules to it. Its accuracy is therefore exactly as good as your computer or phone's clock — which, on any modern device, is kept in sync with atomic-clock time over the internet to within a fraction of a second. There's no server round-trip and nothing to refresh: the display updates every second right in your browser. If a city ever looks wrong, the fix is almost always to check that your own device's clock and time-zone setting are correct, since every card is calculated from that single local reference.