How daylight saving time actually works
Twice a year, on a Sunday in spring and a Sunday in fall, hundreds of millions of clocks jump by an hour. Most people remember the rule ("spring forward, fall back") but skip the mechanic. Here's what actually happens — and why some software still breaks every time.
If you just want to know when the next clock change is for your country, the DST calendar has the dates. For everything else, read on.
The basic mechanic
In spring, your local clock skips forward by one hour at a designated moment. In the United States this happens at 2:00 AM local time on the second Sunday of March. The clock jumps from 1:59:59 directly to 3:00:00. The hour from 2:00 to 2:59 doesn't exist that day in your zone.
In fall, the reverse: at 2:00 AM on the first Sunday of November, the clock falls back to 1:00. The hour from 1:00 to 1:59 happens twice that day. There are two 1:30 AMs.
That's the entire mechanic. Everything else — sleep effects, software bugs, political fights — flows from those two facts.
Why 2 AM?
Two reasons, both practical.
Most people are asleep. In 1966, when the US Uniform Time Act standardized the rule, the Department of Transportation picked 2 AM because it minimized disruption. Bars are closed. Trains are mostly not running. Shift changes haven't happened. If you have to skip an hour, do it when no one's looking.
It avoids the date boundary. If the spring-forward happened at midnight, the date itself would be ambiguous. By picking 2 AM, the date stays clean — March 13 just has a missing hour, not a missing or duplicate date.
The European Union picked 1 AM UTC for the same reason: less to break. Australia picks 2 AM in the southern hemisphere's spring (October). Most countries that observe DST picked some quiet hour for the same logic.
The "missing hour" and the "duplicate hour"
In spring, an hour disappears. In fall, an hour repeats. Both create real problems.
The missing hour
Anything scheduled to happen at 2:30 AM on a spring-forward Sunday in the US doesn't happen. Your alarm clock skips it. Your `cron` job that's supposed to run at 2:30 AM either doesn't fire or fires at 3:30 AM, depending on the cron implementation. If you booked a flight that supposedly takes off at 2:15 AM that morning, you've booked a flight that doesn't exist — airlines avoid scheduling departures during the missing hour for exactly this reason.
The duplicate hour
In fall, the local time 1:30 AM happens twice. Without further information, you can't tell which one a timestamp refers to. This is a real bug source: a database row stamped "2026-11-01 01:30:00 America/New_York" is genuinely ambiguous unless the offset is also stored.
This is why every time-zone-aware system stores either UTC or "local time + UTC offset." If you've ever wondered why PostgreSQL's `timestamptz` is so insistent about converting to UTC under the hood, this is the reason. The Unix epoch converter shows the same — every epoch second has a unique UTC mapping, never an ambiguous one.
DST around the world
Roughly 70 countries observe daylight saving time. The rest — most of Africa, most of Asia, a chunk of South America — don't.
| Region | Spring forward | Fall back |
|--------|----------------|-----------|
| United States, Canada | 2nd Sunday of March, 2 AM | 1st Sunday of November, 2 AM |
| European Union | Last Sunday of March, 1 AM UTC | Last Sunday of October, 1 AM UTC |
| United Kingdom | Last Sunday of March, 1 AM UTC | Last Sunday of October, 1 AM UTC |
| Australia (NSW, VIC, SA, TAS, ACT) | 1st Sunday of October, 2 AM | 1st Sunday of April, 3 AM |
| New Zealand | Last Sunday of September, 2 AM | 1st Sunday of April, 3 AM |
| Mexico | Abolished in 2022 (most of country) | — |
| Brazil | Abolished in 2019 | — |
| Russia | Abolished in 2011 | — |
| Most of Africa, Asia | No DST | — |
In the US, Hawaii and most of Arizona don't observe DST either. Saskatchewan in Canada also opts out. These exemptions are usually justified by latitude (less to gain in tropical zones) or political preference.
The European DST mismatch
Here's a quirk worth knowing: the EU and the US shift DST on different weekends. The US springs forward on the second Sunday of March; the EU does it on the last Sunday of March. That's usually a 2-week gap.
For 2 weeks every March, the time difference between New York and London is 4 hours instead of the usual 5. For one week between October and November, it's also 4 hours instead of 5 (US falls back later). If you have a recurring transatlantic meeting, those 3 weeks per year are when it shifts an hour relative to your normal experience. The meeting planner handles this automatically.
The software side: why DST keeps breaking things
DST is one of the all-time greatest sources of date-related bugs. A few greatest hits:
- Microsoft, 2007. When the US extended DST by 4 weeks via the 2005 Energy Policy Act, every Outlook calendar entry created during the affected window shifted by an hour. Companies spent weeks fixing meeting times.
- Apple, 2010. A bug in iOS made alarm clocks fire an hour early after Australia's autumn DST shift. People missed flights and meetings.
- Reddit, 2012. A leap second (a different bug class, but related) crashed several services hosted on Linux servers.
- Various cron systems. Cron jobs scheduled for 2:30 AM never fire on spring-forward Sundays in zones where 2 AM is the shift. The cron builder lets you check what your cron expression actually does.
The deep answer to "why does this keep happening" is that local time is not a monotonically increasing function. Every assumption you make about wall-clock time has at least two edge cases per year. Programming languages have learned the lesson — Python's `zoneinfo`, JavaScript's upcoming `Temporal`, Java 8's `java.time` — all handle DST correctly if you use them correctly. The bugs come from older code that was written before these libraries existed.
Why DST exists in the first place
The original argument was energy savings. Benjamin Franklin half-jokingly suggested it in 1784 to save candles. Germany formally adopted it in 1916 during World War I to save coal. The US followed in 1918. The energy argument has since been re-examined many times, with mixed results — modern studies suggest savings are tiny or even negative once you account for air conditioning and lighting in dark mornings.
The argument that's actually held up is recreation. People prefer evening daylight for activities — outdoor dining, sports, gardening, retail shopping. Most countries that have abolished DST have done so by making summer time permanent, not by going back to year-round standard time.
Will DST go away?
Probably yes, eventually, but slowly.
The European Union voted in 2018 to abolish biannual clock changes. Each member state was supposed to choose permanent winter time or permanent summer time. Then COVID happened, every country has different priorities, and the reform stalled. As of 2026 the EU still observes DST.
In the United States, the Sunshine Protection Act has passed the Senate twice (2022, 2024) but failed in the House each time. Several US states have laws on the books to switch to permanent DST, but they require federal approval to take effect. So nothing has changed.
Mexico abolished DST for most of the country in 2022. Brazil abolished it in 2019. Russia abolished it in 2011 (and went through a permanent-summer-time period that was unpopular and eventually reversed to permanent winter time). The trend is toward abolition, but everyone's moving at their own pace.
FAQ
When does DST start in 2026?
In the United States, DST begins on Sunday, March 8, 2026, at 2:00 AM local time (clocks spring forward to 3:00 AM). In the European Union, DST begins on Sunday, March 29, 2026, at 1:00 AM UTC. The full schedule for every country is on the DST calendar.
Why does the EU shift two weeks after the US in spring?
It's a coincidence of how each region picks its dates. The US uses the second Sunday of March; the EU uses the last Sunday of March. Most years that's a 2-week gap. There's no underlying reason — just that the rules were written separately at different times.
Does DST save energy?
The original justification was yes. Modern studies are mixed and often show net-zero or slightly negative savings, because air conditioning loads in summer evenings have grown faster than lighting savings. The US Department of Energy's 2008 study found a savings of about 0.5% — small enough to be within measurement error.
Why do farmers oppose DST? (They actually do, by the way)
There's a folk belief that DST exists "for farmers." It's the opposite: farmers historically opposed DST because their schedules are set by sunrise (when they milk cows, harvest, etc.), not by clocks. DST forced them to deliver products to markets that opened an hour earlier on the wall clock — losing them an hour of cool morning work.
What happens to a flight scheduled during the missing hour?
Airlines avoid scheduling departures during the spring-forward gap. If you see a flight booked for 2:30 AM on the second Sunday of March in the US, the airline almost certainly times it relative to the destination zone or shifts it to 3:30 AM. Train schedules in Europe have specific procedures for handling the duplicate hour in fall — extra-long stops at certain stations to absorb the extra hour.
Why is it called "saving" daylight if the daylight is the same?
The phrase is short for "daylight saving" — saving evening daylight for use, by shifting clocks. The total amount of daylight in a day doesn't change; only the wall-clock label on it does. The original 1916 German law called it "Sommerzeit" (summer time), which is more accurate. Most languages still use a phrase like "summer time" rather than "daylight saving."
Bottom line
DST is one of the simplest rules with the most complex consequences. The clock jumps forward an hour in spring, back an hour in fall, and that single mechanic creates a missing hour, a duplicate hour, software bugs, sleep disruption, and political fights — all of which return every six months on schedule.
If you're trying to figure out the next DST date for your country, the DST calendar lists them all. If you're scheduling a recurring meeting that crosses the EU-US DST gap, the meeting planner handles the offset shifts automatically.