World Clock, Explained
After reading this you will know how the current time in any city is computed from a single global clock, why the offset can change twice a year, and how to read a row of clocks without making an off-by-one-hour mistake.
What a world clock actually shows
A world clock shows the same instant expressed in many places at once. There is only one "now" on Earth. What changes from city to city is how that instant gets written on a wall clock. When it is 18:00 in London, it is 13:00 in New York and 03:00 the next day in Sydney. Same moment, three different labels.
The tool keeps one internal reference time and applies a different rule for each city you add. The reference is Coordinated Universal Time, written UTC. Every city's displayed time is UTC plus or minus a number of hours (and sometimes minutes). New York in winter is UTC-5. Kolkata is UTC+5:30. Kathmandu is UTC+5:45. The offset is the whole story.
UTC is not a time zone anyone lives in by law, but it is the anchor for all of them. Greenwich Mean Time (GMT) is within a fraction of a second of UTC and is often used loosely to mean the same thing.
When to use it, and when not to
Use a side-by-side world clock when you coordinate across places: scheduling a call for a team split between Berlin, Bengaluru and San Francisco, checking whether a shop in Tokyo is open before you email, or watching a market that trades from 09:30 local time in another country.
Do not use a world clock to plan a single fixed future event where daylight saving might shift. If you agree to meet "at 15:00 London time on 30 March," the offset to New York can differ from what you see today, because the two regions switch their clocks on different dates. For a one-off future meeting, pin the event to a named zone and confirm both local times close to the date. The live clock tells you the offset right now, not the offset three weeks from now.
The offset math
The conversion from UTC to any local wall time is a single addition.
Here t_{\text{UTC}} is the current instant on the global clock, t_{\text{local}} is the time you write on the wall in that city, and offset is that city's distance from UTC in hours and minutes. A city east of Greenwich has a positive offset; a city west has a negative one.
The offset itself is not constant. Many regions observe daylight saving time (DST), moving their clocks forward by one hour in spring and back in autumn. So the offset is really a function of the date:
Here \text{offset}_{\text{standard}} is the region's winter offset and \text{DST}(d) is either 0 or +1 hour depending on the date d. New York's standard offset is -5. From mid-March to early November its DST term is +1, giving -4. This is why the gap between London and New York is 5 hours for part of the year and 4 hours for another part.
A worked example with five cities
Take a single instant, 17:00 UTC on a day in January (winter in the northern hemisphere, so no DST there). Apply each city's offset.
Converting 17:00 UTC to five cities
- London in January is on standard time,
UTC+0. So17:00 + 0 = 17:00, same day. - New York is
UTC-5in January. So17:00 - 5 = 12:00, same day. - Los Angeles is
UTC-8. So17:00 - 8 = 09:00, same day. - Kolkata is
UTC+5:30all year. So17:00 + 5:30 = 22:30, same day. - Sydney is
UTC+11in January (its summer, DST active). So17:00 + 11 = 04:00the next day.
Notice Sydney rolled past midnight. The date shifted forward by one calendar day. This is the single most common source of confusion, so the tool shows the date next to each clock, not just the time.
| City | Offset | Local time | Date shift |
|---|---|---|---|
| London | +0:00 | 17:00 | same day |
| New York | -5:00 | 12:00 | same day |
| Los Angeles | -8:00 | 09:00 | same day |
| Kolkata | +5:30 | 22:30 | same day |
| Sydney | +11:00 | 04:00 | next day |
How the offset moves through the year
Because DST turns on and off, the offset between two cities is a step function, not a constant. The clearest way to see it is to fix one city and watch the gap to another across twelve months.
Reading the results without errors
Three fields tell you everything: the time, the date and the UTC offset. Read them together.
- Local time
- The wall-clock time in that city right now, updated every second.
- Date
- The calendar day in that city. It can be one day ahead of or behind your own. Sydney is often "tomorrow" relative to Los Angeles.
- UTC offset
- How far that city sits from UTC at this moment, including any active DST. This is the number you add to UTC to get the local time.
To compare two cities, subtract their offsets. If Tokyo is UTC+9 and Paris is UTC+2 in summer, Tokyo is 9 - 2 = 7 hours ahead of Paris. A 15:00 meeting in Paris is 22:00 in Tokyo the same day.
Common mistakes
Do not assume the offset you see today holds next month. The gap between London and New York is 5 hours in winter but 4 hours during the short windows in March and late October/November when only one of them has switched. Scheduling a recurring call by fixed offset instead of by named zone drifts by an hour twice a year.
A second mistake is ignoring the date field. If Los Angeles reads 23:00 and Sydney reads 18:00, it is tempting to say Sydney is 5 hours behind. It is not: Sydney is 19 hours ahead, on the next calendar day. Always check whether the dates match before you subtract.
A third is confusing a country with a time zone. The contiguous United States spans four zones, from Eastern (UTC-5 in winter) to Pacific (UTC-8). "US time" is not a single value. Add the specific city, not the country.
A fourth is the half-hour and quarter-hour zones. India runs on UTC+5:30 nationwide, Nepal on UTC+5:45, and parts of Australia on UTC+9:30. If you round every offset to a whole hour you will be 30 or 45 minutes off.
Related tools
Once you have the local times lined up, other tools help you act on them. Use the Online Alarm Clock to sound at a chosen wall-clock time, or the Countdown Timer to count down to a call. The Online Stopwatch measures elapsed time with laps, and the Pomodoro Timer structures focused work in rounds. If a colleague quotes a time as 21:00 and you think in 12-hour form, the Military Time Converter turns that into 9:00 PM.
Frequently asked questions
Why does the time gap between two cities change during the year?
Because most regions that observe daylight saving change their clocks on different dates. During the weeks when one city has switched and the other has not, the offset between them is one hour different from usual. London and New York, for example, sit 5 hours apart most of the year but 4 hours apart for a few weeks in spring and autumn.
What is the difference between UTC and GMT?
For everyday purposes they are the same. UTC is defined by atomic clocks and kept within 0.9 seconds of Earth's rotation using leap seconds. GMT is the older astronomical reference at the Greenwich meridian. The two never differ by more than a second, so a world clock treats London in winter as UTC+0.
How can a city be a whole day ahead?
The largest eastward offsets reach UTC+14 and the largest westward reach UTC-12, a spread of 26 hours. When it is late evening on one side of that span it is already the following afternoon on the other. The date field on each clock shows which calendar day applies.
Does the clock update automatically for daylight saving?
Yes. The times use the IANA time zone database in your browser, which knows each region's DST rules and transition dates. When a region springs forward or falls back, the displayed offset changes on its own.
Is any of my data sent to a server?
No. Everything runs locally in your browser. The list of cities you add and the live times are computed on your device, and nothing is transmitted.
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