How Daylight Saving Time Works

Daylight saving time (DST) is the practice of setting clocks forward by an hour for part of the year — typically spring through autumn — and then back again. The goal is to shift an extra hour of daylight into the evening during the months when it’s most useful.

Not every country observes it

DST is far from universal. Most of Asia — including Thailand, Vietnam, and India — does not observe it at all, and neither do most countries near the equator, where day length barely changes across the year. Meanwhile, most of North America and Europe do observe it, though on different calendar rules.

Typical transition rules

• United States and Canada: clocks move forward on the second Sunday of March, and back on the first Sunday of November.
• European Union and United Kingdom: clocks move forward on the last Sunday of March, and back on the last Sunday of October.
• These rules are set independently by each country and can be changed by local legislation at any time.

Because the rules differ by country — and some places don’t observe DST at all — the time difference between two cities can change twice a year even though neither city’s local clock rule ever seems to move for you personally. See our guide on why time differences change for a worked example.