Daylight Saving Time Explained — Who Uses It, When, and Why

What DST actually does

Daylight saving time (DST) shifts the clock forward by one hour during the warmer months, so that daylight lasts later into the evening. A region that keeps UTC−5 in winter keeps UTC−4 in summer; solar noon moves from ~12:00 to ~13:00 on the clock. The traditional mnemonic: "spring forward, fall back."

The idea is usually credited to a 1895 proposal by New Zealand entomologist George Hudson (who wanted more after-work daylight for insect collecting) and was first adopted at national scale by Germany in 1916 to save coal during World War I. Most industrialized countries followed within a few years.

Who observes DST today

Fewer places than you might think — only about a third of the world's countries, and the trend is toward abolishing it:

• North America: The US and Canada observe DST (with exceptions: Arizona, Hawaii, Saskatchewan, and most US territories do not). Mexico abolished nationwide DST in 2022, keeping it only near the US border.
• Europe: All EU countries, the UK, and most neighbors change clocks together (the EU has debated ending the practice for years without acting). Russia, Belarus, Turkey, and Iceland do not observe DST.
• Asia: Essentially none. Korea, Japan, China, India, and Southeast Asia all keep the same time year-round. Iran dropped DST in 2022.
• Southern hemisphere: Parts of Australia (NSW, Victoria, South Australia, Tasmania — but not Queensland or Western Australia), New Zealand, Chile, Paraguay, and a few others observe it during the southern summer.
• Africa and the tropics: Almost nowhere. Near the equator, day length barely changes with the seasons, so there's no evening daylight to "save".

When the clocks change

The dates are set by law and differ by region — which means the northern world changes clocks on different weekends, and the southern hemisphere changes in the opposite direction:

Notice the gaps: for roughly two to three weeks each spring and autumn, the US has changed its clocks but Europe hasn't (or vice versa). During those windows, the usual New York–London difference of 5 hours temporarily becomes 4.

How DST changes international time differences

This is the part that catches everyone. The time difference between two cities is not a constant — it depends on which of them is currently observing DST:

• Seoul ↔ New York: 14 hours in winter, 13 hours in summer (Seoul never changes; New York does).
• London ↔ Sydney: 11 hours in the northern winter, 9 hours in the northern summer — a two-hour swing, because the two cities shift in opposite seasons.
• New York ↔ London: normally 5 hours, but 4 hours during the spring and autumn mismatch windows.

Any recurring international meeting will be hit by this at least twice a year. A 9 a.m. New York call that lands at 11 p.m. in Seoul becomes a midnight call when the US falls back in November — unless someone notices. (Our guide to scheduling across time zones covers how to defuse this.)

Does DST actually save energy?

The original rationale — saving fuel for lighting — has weakened as lighting became a small slice of energy use. Modern studies are mixed: some find small electricity savings, others find increases (more evening air conditioning and driving). Research has also documented short-term upticks in heart attacks, workplace accidents, and traffic collisions in the days after the spring change, attributed to lost sleep. This evidence is why many countries have abolished DST in recent years and why both the EU and US have repeatedly debated ending it.

How software keeps up

DST rules are legislation, and legislatures change them — sometimes with only weeks of notice. All current and historical rules live in the IANA Time Zone Database, which operating systems and browsers update regularly. That's why gworldtime maps every city to its IANA zone identifier rather than a fixed offset: when the rules change, the clock follows automatically. You never need to remember whether a city is currently on summer time — the globe just shows the right answer. (More on this in how time zones work.)

Practical survival tips

1. Never store or memorize a fixed time difference between two cities. Check it live, close to the date you need it.
2. Beware the mismatch windows (mid-March and late October/early November) when the US and Europe are out of sync.
3. Schedule recurring international meetings anchored to one named city ("9:00 a.m. New York time"), so everyone else's local time absorbs the shift predictably.
4. Double-check anything scheduled at 1–3 a.m. local time on a change date — that hour may not exist (spring) or may occur twice (autumn).

Open the World Clock →

Read next: How time zones work · UTC vs GMT · DST dates for 2026–2027