Daylight saving time explained

By Adam Gouda · Last reviewed November 9, 2026

Daylight saving time (DST), called “summer time” in much of the world, moves clocks forward one hour in spring and back one hour in autumn, shifting an hour of daylight from early morning to evening. It sounds simple, but because countries choose different rules and dates, it’s one of the biggest sources of confusion in world time.

When clocks change

The current U.S. schedule was set by the Energy Policy Act of 2005 and took effect in 2007. Europe changes all its clocks at the same instant, 01:00 UTC, regardless of local zone.

Places that don’t use daylight saving time

• Most of the world’s population. Much of Asia, Africa and South America doesn’t change clocks. Examples include Japan, China, India, Singapore and the Gulf states.
• In the U.S.: Hawaii and most of Arizona (the Navajo Nation in Arizona does observe DST).
• Mexico ended daylight saving time for most of the country in 2022, except areas near the U.S. border.
• Russia and Turkey stay on permanent year-round time.

Egypt is an example of how rules change: it suspended daylight saving time for several years and reinstated it by law in 2023.

Why the gaps between cities change during the year

Because the U.S. and Europe switch on different weekends, the time difference between, say, New York and London is five hours most of the year but four hours for a few weeks in March and again in late October and early November. Countries that don’t observe DST shift by an hour relative to everyone who does. A meeting at the same local time in two cities can suddenly be an hour off.

UTC never changes

This is a key reason aviation and the military use Zulu time. UTC is the same all year, so a time written in UTC is never ambiguous, even in the hour when clocks “fall back” and 1:30 a.m. happens twice.

Why does daylight saving time exist?

The idea is to make better use of evening daylight and, originally, to save energy. It was widely adopted during the World Wars. Whether it saves meaningful energy today is debated, and some countries and U.S. states have discussed staying on one time year-round. Rules can change with little notice, which is why software relies on regularly updated databases like the IANA tz database.

How the Global Flight Clock handles it

Each clock on the Global Flight Clock uses your browser’s built-in time zone data, so daylight saving changes for each city are applied automatically. The time zone label under each clock, such as EST or EDT, shows which one is currently in effect.

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