Assumptions
- Offsets and daylight-saving rules come from the IANA database inside your browser. A country that changes its rules shows up here once the browser is updated.
- Cities that share a time zone are shown once. Some countries use more than one zone.
- Working hours are taken as 9 to 18. They differ by country and industry.
- The hour or two around a daylight-saving change is special. Times that do not exist are flagged; times that happen twice are read as the first of the two.
- The cities and times you enter never leave your browser.
How it is calculated
- Your clock time is turned back into an actual instant, and that instant is then read off as the clock time in the other city.
- The offset itself depends on the instant. So the offset is worked out again from the first result and applied a second time. Without that second pass, times near a daylight-saving change land an hour out.
- Everything is computed in minutes, because offsets that are not whole hours genuinely exist.
- A change of date is called out. Reading the hour and missing the date is the classic time zone mistake.
Common mistakes
- Check again during a changeover week. Countries switch on different dates: the EU on the last Sunday in March, the US on the second Sunday. For three weeks in between, the usual difference is an hour out.
- Rule changes need a browser update. Mexico and Egypt have both altered their daylight-saving arrangements in recent years.
- The working-hours colours are a rough guide. Some countries take a long lunch; some finish early on Fridays.
FAQ
Does it account for daylight saving?
It does, and that is the reason to use it rather than doing the arithmetic yourself. New York is 14 hours behind Seoul in winter and 13 hours behind in summer. Anyone who memorises one number gets it wrong twice a year. Set the date and this uses the offset that actually applied on that date.
Where does the time zone data come from?
From the IANA time zone database that ships inside your browser and operating system. The daylight-saving start and end dates are in there too, so nothing is downloaded and nothing is looked up remotely. The cities and times you enter never leave the page.
Do half-hour and quarter-hour offsets work?
Yes. India is UTC+5:30, Nepal +5:45, and Newfoundland −3:30. Treating offsets as whole hours puts those places out by half an hour. This works in minutes, and the city list deliberately includes them.
It says the time does not exist
On the day daylight saving begins the clocks skip an hour. In the US Eastern zone the clock goes straight from 2am to 3am on the second Sunday in March, so 2:30am does not exist that day. Quietly converting a time that never happened is how meetings end up an hour out, so it tells you instead.
How do I read the meeting grid?
The top row is each hour of your own day, 0 to 23. Each row below shows what that hour is in another city. Read down a column: where the greens line up, everyone is inside working hours. Yellow means early morning or evening — possible, but someone is stretching.
Why only these cities?
Not every city in the world, but the 57 places the people using this site are likely to be calling. Cities that share a time zone are listed once: Madrid and Barcelona show the same clock, so they are one row.
Why do some US states not change their clocks?
Arizona (outside the Navajo Nation) and Hawaii stay on standard time all year, which is why Phoenix and Honolulu are listed separately. During the summer Phoenix matches Los Angeles rather than Denver, which catches people out.