Daylight Saving Time (DST) is the practice of moving clocks forward by one hour in spring and back by one hour in autumn, shifting an hour of daylight from morning to evening during the warmer months.

Where the idea came from

The modern concept is usually credited to New Zealand entomologist George Hudson, who proposed a two-hour shift in 1895 to give himself more daylight after work to study insects. Germany and Austria-Hungary were the first countries to implement it nationally, in 1916, as a wartime measure to conserve coal. Other countries, including the UK and the US, followed shortly after.

Which countries observe DST today

Most of Europe, North America, and parts of the Middle East and Oceania still observe seasonal clock changes. Notably, most of Asia, Africa, and countries near the equator do not, since the benefit of shifting daylight is smaller when day length barely changes across seasons. Arizona (outside the Navajo Nation), Hawaii, and most of Russia and China have also opted out of seasonal changes entirely.

When do clocks change?

In the United States and Canada, clocks typically spring forward on the second Sunday in March and fall back on the first Sunday in November. In the European Union and the UK, the shift happens on the last Sunday in March and the last Sunday in October. Because these dates don't always align, there are usually one or two weeks each spring and autumn when the time difference between, say, New York and London is temporarily different from usual.

The ongoing debate

Supporters argue DST reduces energy use and gives people more usable daylight in the evening for outdoor activities. Critics point to research suggesting the energy savings are minimal in modern, air-conditioned economies, and that the twice-yearly clock change disrupts sleep patterns and is linked to short-term upticks in traffic accidents and heart attacks. Several regions, including the European Union, have discussed ending the practice altogether, though a coordinated switch to permanent standard or summer time has not yet been finalized.

Practical tip

If you're scheduling a call across time zones near a DST transition date, double-check both locations individually rather than assuming a fixed offset — the gap between two cities can shift by an hour for a week or two if their DST start and end dates don't match.