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GMT stands for Greenwich Mean Time, the historical reference clock kept at London's Greenwich observatory.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.