What is the difference between GMT and UTC?

GMT is a historical time standard based on mean solar time at Greenwich. UTC is the modern civil time scale used for offsets (UTC±HH:MM). For everyday clocks they usually agree to the second; aviation and science prefer UTC.

Why is not every zone exactly one hour wide?

Politics, islands, and trade links reshape borders. Some places use half-hour or 45-minute offsets. Longitude still explains the rough pattern; laws define the exact offset.

Do all countries use daylight saving?

No. Many equatorial and some temperate countries keep a single offset year-round. When DST exists, start and end dates differ by jurisdiction and change over time—another reason to trust IANA rules.

How should I plan a meeting across zones?

Pick a concrete local time in one city, convert it with the converter, and double-check DST near transition weekends. Difference pages show the current hour gap between two places.

What are time zones?

A time zone is a region that agrees to use the same civil clock. Offsets from Coordinated Universal Time (UTC), daylight saving rules, and history explain why noon in Tokyo is not noon in New York.

Last updated 2026-08-04 · Written by the NowZones publisher in Denmark. Offsets and DST examples use the IANA Time Zone Database via the same libraries as the city clocks. This is editorial explanation, not legal, medical, or aviation advice.

Shared civil time for a region

Before standard zones, towns set clocks by local solar noon—when the sun was highest. That worked for walking distances, but broke once trains and telegraphs connected distant places on the same schedule.

Today, each place maps to an IANA time zone id such as America/New_York or Europe/Copenhagen. The id carries the standard offset and the daylight saving rules for that region—not just a country name.

Earth turns 360° in about 24 hours, so each 15° of longitude is roughly one hour of clock difference from the prime meridian.

How the offset is calculated

Idealised zones follow longitude: divide the globe into 24 bands of 15°. The zero meridian through Greenwich defines UTC±00:00. Moving east adds hours (UTC+); moving west subtracts (UTC−).

Real borders follow politics and geography, so offsets are often whole or half hours that do not match pure longitude. Example: if it is 12:00 UTC, a place at UTC+09:00 shows 21:00 the same calendar day.

Civil time = UTC plus or minus a fixed offset (until daylight saving temporarily shifts the offset).

Daylight saving time

Many regions shift clocks forward in warmer months so evenings stay lighter. The IANA zone still identifies the place; only the active UTC offset changes for part of the year.

Difference pages and city clocks on NowZones show whether daylight saving is in effect and how that changes meeting math. Always check both sides of a pair near transition weekends.

A short history of standard time

Railways in the 19th century forced national standard times. The 1884 International Meridian Conference in Washington, D.C. recommended a prime meridian at Greenwich and a worldwide hour system—foundations of modern zones.

Greenwich Mean Time (GMT) was long the reference. Coordinated Universal Time (UTC) later became the civil standard, kept close to Earth’s rotation with leap seconds. Software today relies on the IANA Time Zone Database (tzdb) for historical and future rule changes.

From local solar time to railway schedules, the 1884 meridian conference, GMT, UTC, and the IANA tzdb used by modern clocks.

How NowZones uses time zones

NowZones computes city times and conversions with the IANA Time Zone Database via well-tested libraries. Pages expose the IANA id, current offset, and daylight saving status—not a decorative clock alone.

Use the converter for meeting times, browse city clocks for live local time, or open a difference page when you need “when it is 09:00 there…”.

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Frequently asked questions

What is the difference between GMT and UTC?: GMT is a historical time standard based on mean solar time at Greenwich. UTC is the modern civil time scale used for offsets (UTC±HH:MM). For everyday clocks they usually agree to the second; aviation and science prefer UTC.
Why is not every zone exactly one hour wide?: Politics, islands, and trade links reshape borders. Some places use half-hour or 45-minute offsets. Longitude still explains the rough pattern; laws define the exact offset.
Do all countries use daylight saving?: No. Many equatorial and some temperate countries keep a single offset year-round. When DST exists, start and end dates differ by jurisdiction and change over time—another reason to trust IANA rules.
How should I plan a meeting across zones?: Pick a concrete local time in one city, convert it with the converter, and double-check DST near transition weekends. Difference pages show the current hour gap between two places.