Twice a year, large parts of the world move their clocks forward or back by an hour. The practice is called daylight saving time, or DST, and it shifts the clock so that daylight hours line up more closely with when people are awake and active, rather than changing how much daylight actually exists.
The mechanics are simple, but the rules for exactly when the switch happens vary by country, and that variation is a common source of scheduling mistakes and software bugs.
The basic idea: spring forward, fall back
In spring, clocks are set forward by one hour, which means the clock skips directly from, for example, 01:59 to 03:00. That day is one hour shorter. In autumn, clocks are set back by one hour, so a moment near the transition, such as 01:30, occurs twice. That day is one hour longer. The offset from UTC changes by exactly one hour during the transition, while the underlying UTC time keeps moving forward without interruption.
The intent behind the practice is to shift daylight into the evening during the months when days are naturally longer, on the reasoning that this reduces the need for artificial lighting in the hours after most people finish work.
Who observes it, and when
Daylight saving time is not universal. Most of Europe and North America observe it, while most of Asia, Africa, and equatorial regions do not, because the length of daylight there varies little across the year.
- United States and Canada: clocks move forward on the second Sunday of March and back on the first Sunday of November.
- European Union: clocks move forward on the last Sunday of March and back on the last Sunday of October, with the change taking effect at 01:00 UTC.
- Most of Asia, Africa, and much of South America: no daylight saving time is observed at all.
Because the US and EU switch on different dates, there are a few weeks each spring and autumn when the time difference between, say, New York and London is not the usual number of hours.
Why the dates and rules keep changing
Governments periodically revise their daylight saving policies, sometimes adjusting the start or end dates, and occasionally proposing to abolish the practice altogether. Because these rules can change with little notice, software cannot simply hardcode an offset for a country. Instead, operating systems and programming languages rely on the IANA time zone database, a continuously updated set of rules covering every observed zone, to compute the correct offset for any date automatically.
Where the switch causes real problems
The hour that is skipped in spring does not exist as a wall-clock time, and the hour that repeats in autumn is ambiguous unless you also know the offset. Calendar invitations, cron jobs, and flight bookings that are naively scheduled for a local time during the transition window can land at the wrong moment if the software does not account for the change. This is one of the reasons UTC is preferred for backend timestamps: it has no transitions to worry about.
If you are coordinating a call across zones during a transition period, it is worth checking both sides against a live reference. Our world clock reflects each city's current offset automatically, including during DST changeovers, and our guide to time zone abbreviations explains how letters like EST and EDT shift depending on the season.
A brief note on origins
The idea of shifting clocks seasonally is often associated with proposals from the late 19th and early 20th centuries aimed at making better use of daylight during wartime fuel shortages. It was adopted and abandoned repeatedly through the 20th century by different countries before settling into the patterns most regions follow today, and debate over whether to keep it continues in several jurisdictions.