Time Zones Explained
Why the world has time zones, how UTC offsets work, and what daylight saving changes.
Why time zones exist
The sun is overhead at different moments around the world, so each region sets its clocks so that midday is roughly when the sun is highest. Time zones group places that keep the same clock time.
UTC offsets
Every zone is written as an offset from UTC, the world's reference clock. New York in winter is UTC−05:00 (five hours behind UTC); India is UTC+05:30. Most offsets are whole hours, but India, Iran, Afghanistan, Nepal and parts of Australia use 30 or 45 minute offsets.
Zone names vs offsets
A zone name like Eastern Time covers two offsets: EST (UTC−05:00) in winter and EDT (UTC−04:00) in summer. That is why "ET" is safer than "EST" in a summer meeting invite: EST in July is one hour off.
Daylight saving time
Many countries move their clocks forward an hour in spring and back in autumn. Not everyone does, and they don't all change on the same day, so the gap between two cities can change for a few weeks a year. See the next clock changes.
The time zone database
Computers use the IANA time zone database, which names zones by city, such as America/New_York or Asia/Kolkata, and records every rule change. Our clocks use the copy built into your browser.
Questions
How many time zones are there?
There are 24 hour-wide bands in theory, but around 38 offsets are in use today, because some zones are 30 or 45 minutes off and some go up to UTC+14:00.
What is the difference between UTC and GMT?
For everyday use they show the same time. UTC is the scientific standard; GMT is the time zone the UK uses in winter.
Why do some countries not use daylight saving time?
Near the equator, day length barely changes through the year, so moving the clocks brings little benefit.
By the TimeNowTools team · Last reviewed 18 October 2026