Daylight saving time, explained

Twice a year, clocks in much of North America, Europe and parts of the southern hemisphere jump forward or back by an hour. This is daylight saving time (DST), also called summer time. It is simple in principle and surprisingly easy to get wrong when you work with people in other countries.

What it is

In spring, clocks move forward one hour, so the sun rises and sets an hour later by the clock. In autumn they move back again. The idea is to shift daylight from the early morning, when many people are asleep, to the evening, when they are awake. London, for example, is on UTC+0 in winter and UTC+1 in summer; New York moves between UTC−5 and UTC−4.

Who uses it, and who doesn't

Most of Europe and North America use DST, as do parts of Australia, New Zealand, Chile and a few other countries. Most of Asia and Africa do not: Japan, South Korea, China, India, Singapore and most countries near the equator keep the same time all year. Near the equator there is little to gain, because the length of the day hardly changes through the year.

Countries also change their minds. Russia stopped changing clocks in 2011, Mexico ended DST in most of the country in 2022, and other countries regularly debate it. That is why reliable clocks use the IANA time zone database, which records these decisions city by city.

When clocks change

• European Union and UK: forward on the last Sunday of March, back on the last Sunday of October, at the same moment everywhere (01:00 UTC).
• United States and Canada: forward on the second Sunday of March, back on the first Sunday of November, at 02:00 local time.
• Southern hemisphere: the seasons are reversed, so Sydney moves forward in early October and back in early April.

Because Europe and North America change on different dates, there are a few weeks each spring and autumn when the gap between, say, London and New York is four hours instead of the usual five.

The mistakes it causes

• Recurring meetings drift. A weekly call set at 9:00 in New York lands at 14:00 in London most of the year, but at 13:00 during the weeks when only one side has changed.
• Some local times don't exist. On the spring-forward night, times between 02:00 and 03:00 never happen in the US. In autumn, the hour between 01:00 and 02:00 happens twice.
• "No DST" countries still move. Tokyo never changes its clocks, yet its difference from Berlin is eight hours in winter and seven in summer, because Berlin moves.

How to avoid them

Schedule in the time zone of the person who has the least flexibility, and check the date, not just the time: a difference that is right today can be wrong in three weeks. Every city page on the world clock shows its current daylight saving period, and the meeting planner applies the rules for the exact date you pick.

More guides: How to work out a time difference · Scheduling meetings across time zones · Reading the day and night map