How daylight saving time works

By Niraj Kumar · Published October 28, 2026

Daylight saving time moves the civil clock forward by an hour for part of the year so that evenings are lighter. The idea is simple; the mechanics are where the trouble lives, because moving a clock forward deletes an hour of local time and moving it back creates a duplicate one. The daylight saving tool gives the next change in any zone, and the DST changes page lists the whole year; this guide explains what those dates mean for anyone who has to schedule across them.

The skipped hour

When DST begins, clocks jump forward. In the United States this happens at 02:00 local time on the second Sunday of March: the clock reads 01:59:59 and then 03:00:00. The times from 02:00 to 02:59 do not occur on that date. An alarm set for 02:30 either fires at 03:30, fires at 01:30, or does not fire at all, depending on the software. A meeting written down as 02:30 on that day cannot be converted to an instant without a decision about what was meant, and the time zone converter says so rather than guessing.

The repeated hour

When DST ends, clocks go back, and the hour before the change happens twice. In the US at 02:00 on the first Sunday of November the clock returns to 01:00, so 01:30 occurs once in daylight time and again an hour later in standard time. A local time in that window is ambiguous: it names two instants an hour apart. Logs written in local time without an offset become impossible to order for that hour, which is one of the strongest arguments for recording timestamps in UTC.

Days that are not 24 hours

The transition days are 23 and 25 hours long. Anything that treats “a day” as 24 hours is wrong on those dates: a job scheduled for “24 hours after the last run” drifts, a daily countdown is an hour off, a time-sheet that subtracts clock-out from clock-in across the change gives the wrong shift length. Calendar arithmetic has to be done on dates, not on seconds, to survive these days, which is why the calculators on this site work on civil dates and only convert to instants at the end.

Who changes, and when

Most of Europe changes together: forward on the last Sunday of March and back on the last Sunday of October, both at 01:00 UTC, so that every EU country switches at the same instant even though the local time reads 02:00 in London and 03:00 in Helsinki. The United States and Canada change on the second Sunday of March and the first Sunday of November at 02:00 local time, zone by zone, so the change sweeps across the continent over four hours. Mexico abolished seasonal changes in 2022. In the southern hemisphere the dates reverse: most of south-eastern Australia goes forward on the first Sunday of October and back on the first Sunday of April, New Zealand on the last Sunday of September and the first Sunday of April, Chile in September and April on dates its government sets year by year.

Most of the world does not change at all. Nearly all of Asia and Africa keep one offset all year, as do Russia since 2014, Turkey since 2016, Brazil since 2019 and Japan since 1951. Within countries that do change, exceptions exist: Arizona (outside the Navajo Nation) and Hawaii do not observe it; Queensland, Western Australia and the Northern Territory do not, while the rest of Australia does; Saskatchewan keeps standard time all year while its neighbours change.

Unusual variants

Lord Howe Island, between Australia and New Zealand, shifts by thirty minutes rather than an hour, from UTC+10:30 to UTC+11. Morocco runs permanent UTC+1 but drops back to UTC+0 for the month of Ramadan, which moves eleven days earlier each year, so its transition dates follow the Islamic calendar. In 2019 the European Parliament voted to abolish the change, and the question has not been resolved since. Any of these rules can change at short notice: in recent years countries have announced changes weeks before they took effect, which is why zone rules are a database rather than a formula, and why the time zone database guide matters to anyone writing software.

The mismatch weeks

Because the US and Europe change on different dates, the gap between them is wrong for two or three weeks in spring and one in autumn. London is normally five hours ahead of New York. From the second Sunday of March to the last Sunday of March it is four, because New York has moved and London has not. From the last Sunday of October to the first Sunday of November it is four again, because London has moved back and New York has not. Standing weekly meetings between the two cities move by an hour for one side during those windows, and this is the single most common cause of a missed transatlantic call. Australia complicates it further: Sydney is normally 11 hours ahead of London in the northern winter and 9 in the northern summer, with a few weeks of 10 at each change.

The zone pages for EST, CET and MST give the next change for each, and say where each label is in force at the moment.

Writing times that survive the change

Three practices avoid nearly all DST problems. Record instants, such as when something happened, in UTC or with an explicit offset. Record future local events, such as a 9 a.m. meeting next spring, as a local time plus an IANA zone name, since the offset in force on that date may not be the one in force today. And never express a recurring event as a fixed number of seconds from the previous one. The guide to storing dates in software goes through the reasoning in detail.

Why the idea persists

The arguments for and against are old and unresolved. Supporters point to lighter evenings, lower lighting demand and more time outdoors after work; critics point to disrupted sleep, a measurable rise in accidents in the days after the spring change, and energy savings that modern studies find small or absent. Several jurisdictions have moved to permanent standard time or permanent summer time rather than keep switching. Whatever the outcome of those debates, the rules for the next few years are published, and they are the rules this site applies.