The basic idea
Every time zone has a standard time with a fixed offset from Coordinated Universal Time (UTC). Daylight saving time (DST) adds one hour to that offset for part of the year. Nothing happens to the actual passage of time: the change is a relabeling of the clock that makes local time one hour later than it would otherwise be.
For example, US Eastern Time is UTC−5 in winter (EST) and UTC−4 in summer (EDT). Central Europe is UTC+1 in winter (CET) and UTC+2 in summer (CEST). The United Kingdom is UTC+0 in winter (GMT) and UTC+1 in summer (BST).
The spring change
When DST starts, the clock skips forward one hour. In the United States this happens at 2:00 local time on the second Sunday in March:
| Real elapsed time | Clock shows |
|---|---|
| 1:58 | 1:58 EST |
| 1:59 | 1:59 EST |
| one minute later | 3:00 EDT |
The hour from 2:00 to 2:59 does not exist that night, so the day has only 23 hours. This is the “spring forward” half of the famous phrase spring forward fall back.
The autumn change
When DST ends, the clock goes back one hour. In the United States this happens at 2:00 local daylight time on the first Sunday in November. The clock reaches 1:59 EDT, then shows 1:00 EST, and runs through the 1:00 hour a second time. That day has 25 hours.
Because 1:30 happens twice, a local time alone is ambiguous on that night. Systems that need precision, such as airlines, banks and servers, record times in UTC or add the offset (for example 01:30−04:00 and 01:30−05:00) to tell the two apart.
Who changes when
The rules differ from place to place:
- United States and most of Canada: forward on the second Sunday in March, back on the first Sunday in November, at 2:00 local time.
- European Union and United Kingdom: forward on the last Sunday in March, back on the last Sunday in October, at 01:00 UTC in every country at once.
- South-east Australia: forward on the first Sunday in October, back on the first Sunday in April.
- New Zealand: forward on the last Sunday in September, back on the first Sunday in April.
Because North America and Europe use different dates, the gap between cities such as New York and London shrinks from five hours to four for a few weeks each spring and autumn. The daylight saving time page shows the current status and next change for each region, and the 2026 clock changes page lists exact dates.
Not always one hour
Almost every place that uses DST shifts by exactly one hour, but there are exceptions. Lord Howe Island in Australia moves its clocks by only 30 minutes. Some places have used two-hour shifts in the past, such as British Double Summer Time during World War II.
How devices know when to change
Computers and phones keep time internally in UTC, which never changes for DST. To show local time, they look up your zone in a time zone database, most often the IANA tz database. It records each zone’s offsets and every DST rule, past and announced. When a government changes its rules, the database is updated and the update reaches devices through operating system and browser updates.
This is why devices usually switch at the right moment even when offline, and why a clock that is set by hand, such as an oven or a car dashboard, often has to be adjusted manually.
Practical effects
- Sleep: the spring change takes an hour of sleep; the autumn change gives one back.
- Schedules: a recurring meeting set in one zone can move by an hour for people in another zone during the weeks when their rules differ. The meeting planner helps catch this.
- Pay and logs: a night shift that spans the change works one hour less in spring and one hour more in autumn.
- Sunrise and sunset: both move one hour later on the clock, though the sun itself is unaffected.
To see how a time compares across zones on any date, try the time difference tool.