The world is divided into 24 standard time zones, each roughly 15 degrees of longitude wide. The prime meridian (0°) at Greenwich, London, establishes Coordinated Universal Time (UTC), from which all other zones are offset.
How Time Zones Work
As the Earth rotates 360° in 24 hours, each 15° equals one hour. Zones east of UTC are ahead (UTC+1 to UTC+14), while those west are behind (UTC-1 to UTC-12). Some regions use half-hour or 45-minute offsets, such as India (UTC+5:30) and Nepal (UTC+5:45).
Daylight Saving Time (DST)
Many countries adjust clocks forward by one hour during warmer months. In the UK, this is British Summer Time (BST, UTC+1), while standard time is Greenwich Mean Time (GMT, UTC+0). Most of Asia, Africa, and South America do not observe DST.
Key Time Zones for Business
EST/EDT (UTC-5/-4) — New York, Toronto, Miami
CST/CDT (UTC-6/-5) — Chicago, Houston, Mexico City
PST/PDT (UTC-8/-7) — Los Angeles, San Francisco, Seattle
GMT/BST (UTC+0/+1) — London, Dublin, Lisbon
CET/CEST (UTC+1/+2) — Paris, Berlin, Rome
JST (UTC+9) — Tokyo, Osaka
AEST/AEDT (UTC+10/+11) — Sydney, Melbourne
Frequently Asked Questions
What is UTC and how is it different from GMT?
UTC (Coordinated Universal Time) is the modern time standard used worldwide. GMT (Greenwich Mean Time) is a time zone. In practice, both show the same time, but UTC is the standard used by aviation, computing, and international communication.
How many time zones are there in the world?
There are 24 standard time zones, but due to half-hour and 45-minute offsets, there are actually 38 unique offsets in use globally, ranging from UTC-12 (Baker Island) to UTC+14 (Line Islands, Kiribati).
Which country has the most time zones?
France has 12 time zones (due to overseas territories), followed by Russia with 11. The United States has 6 (or 9 including territories).
Why do some time zones have 30 or 45 minute offsets?
Some countries chose offsets that better match their geographic position relative to the sun. India uses UTC+5:30, Iran uses UTC+3:30, and Nepal uses UTC+5:45.
What is the International Date Line?
The International Date Line (IDL) is at roughly 180° longitude. Crossing it west advances one calendar day; going east goes back one day. It zigzags to keep island nations in the same day.