How time zones work
The Earth turns 360 degrees in about 24 hours, or 15 degrees of longitude each hour. In theory that gives 24 bands, each one hour apart. In practice, governments draw time-zone borders to follow national and regional borders, so zones are irregular and some places use offsets of 30 or 45 minutes.
UTC and GMT
Coordinated Universal Time (UTC) is the world's reference time. It is kept by atomic clocks, and every local time is written as an offset from it, such as UTC+03:00 for Riyadh. Greenwich Mean Time (GMT) is the older standard based on the sun at the Royal Observatory in Greenwich. For everyday use GMT and UTC match, but UTC is the precise scientific standard. The United Kingdom uses GMT in winter and moves one hour ahead (BST) in summer.
Daylight saving time
Some countries move their clocks forward in spring and back in autumn to make better use of evening daylight. The dates differ between countries, and the Northern and Southern Hemispheres change in opposite seasons. So the gap between two cities can change by an hour for a few weeks each year. This tool uses your browser's IANA time-zone database, so each clock shows the correct offset for its own date, including past rule changes.
The International Date Line
Somewhere on Earth, the calendar has to change. The International Date Line runs roughly along 180° longitude through the Pacific Ocean. When you cross it heading west, you skip ahead one day. Heading east, you repeat a day. The line bends around island nations so each country keeps one date. That is why Kiribati can be a full day ahead of Hawaii even though they are in the same part of the ocean. When cities you compare fall on different dates, the clock cards mark it with +1 day or −1 day.
Why countries keep different local times
Local time is a political choice as well as a geographic one. China spans about five theoretical zones but uses a single national time. Russia and the United States use several zones. India uses one zone with a half-hour offset, and Nepal uses a 45-minute offset. Governments weigh daylight, trade with neighbours and national unity when deciding. That is why GeoCrown never assumes a country has only one time: choose a country above to see each zone found in our city data. For more, read How Time Zones Work.
Time-zone identifiers follow the IANA Time Zone Database (iana.org/time-zones). Day and night are estimated from local clock time (06:00–18:00), not actual sunrise. Times are calculated on your device and depend on its time-zone database being up to date. Reference: Sources.