Daylight Saving Time

What actually happens to the clock

Real time does not stop and does not repeat. The clock on your wall does both, twice a year, and that is the entire subject. These two diagrams are the change itself: the horizontal axis is real elapsed time, the vertical axis is what the clock reads.

In spring the clock jumps forward and the hour in the hatched band never happens. Nobody in that time zone experienced 02:30 that night. That night is 23 hours long.

In autumn the clock falls back and the hour in the marked band happens twice, once at UTC-04:00 and again an hour later at UTC-05:00. That night is 25 hours long.

Two consequences follow, and between them they cause most of the trouble with daylight saving. A local time can fail to exist, so an alarm, a booking or a scheduled job set inside the missing hour has to be resolved by something, and different systems resolve it differently. A local time can happen twice, so a timestamp written as a local time on that night names two different moments unless it also carries its offset.

This is why the rule for storing a time is to store the instant, in UTC or with an offset, and convert only for display. A local time on its own is not always a moment.

The shift is not always an hour. Lord Howe Island moves its clocks by 30 minutes, on 4 October 2026 and 5 April 2026 in 2026.

The next clock changes

The weeks when the usual time difference is wrong

Countries do not change on the same day, so for a few weeks a year the familiar gap between two cities is not the familiar gap. London and New York are usually 5 hours apart.

The southern hemisphere runs the other way. Australia and New Zealand go forward when Europe goes back, so the gap between London and Sydney changes by two hours across the year rather than one.

Who does not change their clocks

Most of the world. Of the 417 zones in the time zone database, 286 do not change their clocks in 2026 and 131 do. The practice is concentrated in Europe, North America and parts of Australasia. Countries near the equator generally do not bother, because there is little seasonal daylight to shift.

Inside countries that do observe it there are exceptions, and they catch people out. Each of these is a zone the database says has no transitions this year.

• Do not change their clocks 286
• Do change their clocks 131
• Zones in the database 417

Counted at build time by probing every zone in the database, not taken from a list.

Arizona is the one that trips people up twice over. It does not change its clocks, so it matches California in summer and Colorado in winter. The Navajo Nation, which extends into Arizona, does keep daylight saving. The time zone database has no separate zone for it, so what the data can show you is the surrounding effect: America/Phoenix has no transitions in 2026, while America/Denver has 2. Driving across the reservation in summer, the observance changes under you.

What the evidence says

This is the part of the subject that has moved from folklore to research, and it is worth separating what is established from what is argued about. Every claim below is linked to its source. Where we have not read a figure ourselves, we do not quote one.

The sleep effect

The American Academy of Sleep Medicine published a position statement in 2020 calling for the abolition of seasonal time changes in favour of permanent standard time, on the grounds that it aligns better with human circadian biology. That is a professional body taking a formal position, and it is the clearest citable statement of where sleep medicine sits. Daylight saving time: an American Academy of Sleep Medicine position statement, Journal of Clinical Sleep Medicine, 15 October 2020 .

The energy argument

Saving energy was the original justification and it is now the weakest part of the case. The two most cited pieces of evidence were published in the same month of 2008 and reach different conclusions: the United States Department of Energy's report to Congress on the impact of extended daylight saving time on national energy consumption , and a study of Indiana, which adopted daylight saving statewide in 2006 . Read them rather than taking a headline figure from anywhere, including us.

Further background on the sleep and circadian effects: Sleep Foundation. The United States rules and their history: NIST.

Why it exists, and the argument about ending it

The idea is simple: move an hour of daylight from the early morning, when most people are asleep, to the evening, when they are not.

The current argument is not really about whether to keep changing the clocks. Most people who want change want to stop changing them. The argument is about which time to stop on, and the two options are genuinely different. Permanent standard time aligns the clock more closely with the sun and is the option the American Academy of Sleep Medicine's position statement supports. Permanent daylight time keeps the extra light in the evening. "Abolish daylight saving" on its own does not say which is meant, and that ambiguity is a large part of why the debate has run so long without resolving.

For developers: the two hours that break software

Historical rules are not the current rules. Transition dates change. This page can show you exactly when, because it asked the database rather than a person: the United States spring transition last moved in 2007. Apply today's rule to a date before that and the offset you get is wrong. Use a library backed by the IANA database, which carries the full history.

An offset is not a time zone. UTC-05:00 is a fact about one moment. America/New_York is a set of rules. Storing an offset and reusing it for another date is wrong every time that date is on the other side of a transition. Adjusting a date object by an offset does not change its time zone, it changes which moment it refers to.

Adding 86,400 seconds is not adding a day. On transition days a day is 23 or 25 hours long. Calendar arithmetic and instant arithmetic are different operations and need different functions.

The general rule: store the instant, keep the time zone identifier alongside anything that has to recur at a local time, and never do calendar arithmetic on a timestamp.

Frequently asked questions