How the world's time zones actually work
Why is the Earth divided into time zones, and why do some countries use half-hour or quarter-hour offsets?
Time zones exist because the Earth rotates once roughly every 24 hours: the sun doesn't rise at the same moment everywhere. The world is loosely divided into 24 bands of 15° longitude, each representing one hour of offset from Greenwich (UTC).
Why do some countries use a half-hour offset? India (UTC+5:30), Iran (UTC+3:30) and parts of Australia (UTC+9:30) chose an intermediate zone so local time better matches the sun's position across their territory, rather than strictly following 15° bands.
What about quarter-hour offsets? Nepal (UTC+5:45) goes even further, as do a few Pacific islands. These are political choices as much as geographic ones.
The International Date Line, which roughly follows the 180th meridian in the Pacific, marks where the date changes by a full day when crossing east or west — which is why some neighboring islands can be up to 24 hours apart in date.
To check the exact time anywhere, use our world clock or the time zone converter.
Watch out for time zone abbreviations. "CST", "IST" or "EST" don't refer to a single zone: for example "CST" can mean either US Central Time or China Standard Time. The IANA zone name (like "Europe/Paris" or "Asia/Tokyo") or an explicit UTC offset removes any ambiguity.
For developers: best practice is to store timestamps in UTC in a database, and only convert to local time when displaying them. This avoids inconsistencies around daylight saving changes or when a government changes its time zone rules.
Handy tools
Find the time slots that work across several time zones at once.
Convert a time from one city to several others at a glance.
The exact dates for the switch to and from daylight saving time in Europe, the United States, Canada and Australia, calculated automatically every year.