World clock

A world clock does two things, and neither needs a server to tell it the time:

• It knows what time it is in UTC. Your phone or computer keeps its clock right by checking time servers over the internet, using the Network Time Protocol, so it stays close to UTC.^[1]
• It applies each place's rules. The browser can show any moment in any IANA time zone.^[2] It looks up each place's offset for that date in the IANA time zone database, the shared record that phones and computers use, and adds it to UTC.^[3]

That's exactly how this board works. It reads your device's clock and the time zone data built into your browser, groups the 90 places on it by their offset right now, and redraws as the minutes pass. If a time looks wrong, check that your device's clock is set to update automatically.

The sun is highest in the sky at different moments around the world, and the Earth turns 15 degrees every hour. So, in principle, places a quarter of the way round the world from each other should be six hours apart. In practice, each country picks its own time:

• China spans about five natural zones but uses one, UTC+8, so the whole country fits in a single row of the board.^[4]
• Spain sits in line with Britain but has been on Central European Time since 1940, which is why Madrid shares a row with Berlin, not London.^[5]
• India chose UTC+5:30 and Nepal UTC+5:45, so their rows sit between the whole hours.^[3]

That's why the board has 36 rows right now rather than a neat 24. To see where the boundaries fall, look at the time zone map, and for how offsets and rules fit together, read how time zones work.

Right now, 32 of the 90 places on the board are on summer time. The board marks them in each row, and warns two weeks before a row's clocks change. Three things make daylight saving time uneven:

• Different dates. The US changes on the second Sunday in March and the first Sunday in November; the EU on the last Sundays of March and October.^{[6, 7]} For a few weeks each year, New York and London are four hours apart instead of five.
• Opposite seasons. Southern places such as Sydney move their clocks forward in October and back in April, the opposite way to Europe and North America.^[8]
• Not everyone takes part. Most of Asia and Africa keep one time all year, and Mexico stopped changing its clocks in 2022.^[9]

The result is that rows split and merge through the year: cities that share a row in January can sit an hour apart in July. The daylight saving time guide has the rules for each region.

• Search in "Add a city" for a city, a country or an IANA time zone name like Europe/Kyiv, and pick it. It's pinned above the board, with its own day track.
• Pin up to eight. Remove one with the cross at the end of its row.
• They're kept in this browser, separately from the cities on the home page, so changing one list never changes the other.
• Share them with "Copy link": the link opens this board with the same pinned cities.

Before a trip, pin your home city and your destination. The two day tracks show at a glance whether a call home will land in someone's night, and the date line under each time tells you when you've crossed into tomorrow or yesterday.

Use "Time shown" to check a moment during your trip, such as your arrival. If either place changes its clocks while you're away, the board warns you two weeks ahead. For jet lag, booking and flight-time tips, read traveling across time zones.

Pin each teammate's city and keep the board open: anyone whose dot sits in the dark part of their track is probably offline. Before proposing a meeting, set "Time shown" to that time and check every pinned city at once, then copy the link so the team sees the same board.

When you need the hours everyone shares, the home page's working-hours chart and the guide to scheduling across time zones go further, including how recurring meetings shift when clocks change.

You rarely need a web API to build a world clock. The device already knows UTC, and every modern browser ships the IANA time zone data. Intl.supportedValuesOf("timeZone") lists the zones it knows,^[10] and Intl.DateTimeFormat shows any moment in any of them.^[2]

const now = new Date();
for (const timeZone of ["America/New_York", "Europe/London", "Asia/Tokyo"]) {
  const time = new Intl.DateTimeFormat("en-US", {
    timeZone,
    timeStyle: "short",
  }).format(now);
  console.log(timeZone, time);
}

• Group by offset at a moment, not by name. Offsets change with daylight saving, so recompute the grouping rather than storing it.
• Keep the data fresh. Time zone rules change every year somewhere; browsers and operating systems ship updates, but servers and old runtimes need patching.^[3]
• Only fetch the time when you can't trust the device. If you need an authoritative clock, sync to a time server over NTP rather than calling a web API on every tick.^[1]

For storing times and handling clock changes, read programming with time zones.

• Time zone converter

  Convert any date and time between two places, with the working shown and daylight saving handled.

• Meeting planner

  Find the best meeting time for up to 10 places, scored hour by hour, with daylight saving handled.

• Meeting cost calculator

  Work out what a meeting costs in people's time, per meeting and per year, from salaries or hourly rates.

• UTC offset calculator

  Find the UTC offset of any place on any date, or every place on an offset, in every notation.

• DST change tracker

  Every clock change worldwide in the next 12 months, your places' next change, and when two places' gap shifts.

• Your cities on the live map

  The home page: your own time next to up to eight cities, with the day and night line drawn on a world map.

1. 1.NIST, NIST Internet Time Service (ITS). Computers keeping their clocks on UTC by asking time servers over NTP.
2. 2.MDN, Intl.DateTimeFormat() constructor. Browsers formatting a moment in any IANA time zone, defaulting to the device's own.
3. 3.IANA, Time Zone Database. Each place's offsets and clock-change rules, and their history.
4. 4.Guinness World Records, Largest country in one time zone. China's single time zone.
5. 5.NPR, Spain's been in the wrong time zone for seven decades. Spain's move to Central European Time in 1940.
6. 6.U.S. Department of Transportation, Daylight Saving Time. US change dates.
7. 7.EUR-Lex, Directive 2000/84/EC on summer-time arrangements. EU change dates, all at 01:00 UTC.
8. 8.NSW Communities and Justice, Daylight saving. Australian change dates and the states that take part.
9. 9.Bloomberg Línea, Mexico abolishes daylight saving time. Mexico ending daylight saving in 2022.
10. 10.MDN, Intl.supportedValuesOf(). Listing every time zone the browser knows.