How we calculate

Everything on this site is calculated in your browser, from public data and published formulas. Here is exactly what we use, and where the limits are.

Current time and time zones

Your device keeps the current moment (in UTC). To turn that into a local time, we ask your browser to format it for a named time zone such as Asia/Seoul or America/New_York, using the standard Intl.DateTimeFormat interface. Browsers ship the IANA time zone database — the same source used by phones, operating systems and servers — which records every country’s offset from UTC and every daylight saving rule, past and planned.

Because the rules live in your browser, updates arrive with browser updates, and nothing about you (not even your time zone) is sent to us.

Limit: the clocks can only be as accurate as your device’s clock. Most devices sync automatically to within a fraction of a second; if yours is set manually it may be off.

Daylight saving time

To find the next clock change for a city, we step through the calendar one day at a time and ask for the city’s UTC offset each day. When the offset changes, we narrow it down to the exact minute. We do this for the current year and the next two, which is how city pages can say “clocks go back 1 hour at 02:00 on Sunday, 1 November”. To detect places that used to change their clocks but stopped (Russia in 2011, Turkey in 2016, Brazil in 2019, Mexico in 2022…), we scan back to the year 2000.

Sunrise, sunset and day length

We use the sunrise equation from the NOAA Solar Calculator family of formulas: the sun’s mean anomaly, equation of centre, ecliptic longitude and declination give the time of solar noon and the hour angle at which the sun’s centre is 0.833° below the horizon (this allows for the sun’s radius and atmospheric refraction). Civil dawn and dusk use 6° below the horizon.

Results are for the city centre at sea level, and are accurate to about a minute. Mountains, tall buildings and unusual weather can shift what you actually see.
Near the poles the sun may not set or rise at all; pages say so rather than show a time.

Day and night on the globe

The globe marks the subsolar point — the place where the sun is directly overhead right now — from the sun’s declination (its latitude, between 23.44° N and 23.44° S through the year) and the equation of time. Each land dot is coloured by the sun’s altitude there: above the horizon is day, down to 6° below is twilight, lower is night. The land outline is Natural Earth data at 1:50m scale via world-atlas, sampled into 5,789 evenly spaced points.

Distances and travel times

Distances are great-circle distances between city centres, calculated with the haversine formula and the Earth’s mean radius of 6,371 km. Travel times divide that distance by a constant speed:

Walking: 5 km/h
Running: 10 km/h
Cycling: 18 km/h
Driving: 90 km/h
Container ship: 37 km/h
Airliner: 870 km/h
Orbital rocket: 28,000 km/h
Speed of light: 299,792 km per second

These are thought experiments — no stops, no roads, no sea lanes — so real journeys take longer.

Cities

The 150 cities are capitals and large cities chosen so that every widely used clock is represented, including half-hour and quarter-hour zones. Names, coordinates, populations and time zones come from GeoNames, licensed under CC BY 4.0. Population is for the city proper as recorded by GeoNames, not the wider metropolitan area.

Working hours and meeting times

Unless you change them, “working hours” mean 9:00 to 18:00 local time. An hour counts as a good meeting hour when every city is inside its window for the whole hour, including cities on half-hour offsets.

Found a problem?

Please tell us the city, the time you saw and the time you expected. We check reports against the IANA database and fix our side quickly.