Why the transition dates are computed, not looked up
No government publishes a list of daylight saving dates decades ahead. What each publishes is a rule, expressed as an ordinal weekday of a month, and every date follows from it. In the United States the rule is 2 U.S.C. §260a as amended by the Energy Policy Act of 2005: daylight saving time runs from 2:00 a.m. local time on the second Sunday in March to 2:00 a.m. local time on the first Sunday in November. In the European Union it is Directive 2000/84/EC: summer time runs from 01:00 UTC on the last Sunday in March to 01:00 UTC on the last Sunday in October.
The rules are stated in weekdays rather than fixed dates for a practical reason. A transition in the small hours of a Sunday morning disrupts the fewest scheduled activities: few flights depart, few shifts change, few markets trade. A fixed calendar date would land on a Tuesday one year in seven.
Because the rule is ordinal, the date moves by a predictable amount each year. It advances one weekday per common year and two per leap year, and then snaps back by a week whenever that would carry it past the ordinal boundary. That is why the US spring transition can be as early as 8 March or as late as 14 March, and never anything else.
The number of days spent on summer time is not constant either. Under the US rule it varies between 238 and 239 days depending on where the two Sundays land; under the EU rule between 209 and 216. If you are amortising a seasonal energy figure, that variation matters.
Finding the nth Sunday of a month
Both forms of the rule reduce to modular arithmetic on the weekday of a known day of the month.
The nth weekday. Take the weekday of the first of the month, call it dow₁, numbered Sunday 0 through Saturday 6. The offset from the 1st to the first occurrence of your target weekday w is (w − dow₁ + 7) mod 7 — the +7 and the mod handle the wrap when the target is earlier in the week than the 1st. Add 7 for each subsequent occurrence. So D = 1 + ((w − dow₁ + 7) mod 7) + 7(n − 1). For the second Sunday in a March that begins on a Sunday: dow₁ = 0, w = 0, so the offset is 0, and D = 1 + 0 + 7 = 8.
The last weekday. Work back from the end. Let L be the number of days in the month and dow_L the weekday of day L. The offset back to the last occurrence of w is (dow_L − w + 7) mod 7, so D = L − ((dow_L − w + 7) mod 7). For March 2026, L = 31 and 31 March 2026 is a Tuesday (dow_L = 2), so the offset is (2 − 0 + 7) mod 7 = 2, and the last Sunday is 31 − 2 = 29 March.
The instant, not just the date. The two rule families differ in an important way. The US and Australasian rules specify a local clock time, so every time zone in the country changes at a different UTC instant — the eastern zone changes three hours before the Pacific zone. The EU rule specifies a single UTC instant, so the whole union changes simultaneously and the local clock time differs by zone: 01:00 in the UK and Portugal, 02:00 in central Europe, 03:00 in Finland and Greece. That is why this calculator asks for your standard offset when you pick a European rule.
Worked example: the 2026 transitions under both rule families
United States, spring 2026. First find the weekday of 1 March 2026. 1 January 2026 is a Thursday. March 1 is ordinal day 60 in a non-leap year, so it is 59 days later, and 59 mod 7 = 3 (7 × 8 = 56). Thursday plus three is Sunday, so dow₁ = 0. Applying the nth-weekday formula with w = 0 and n = 2: D = 1 + ((0 − 0 + 7) mod 7) + 7 = 1 + 0 + 7 = 8 March 2026, at 2:00 a.m. local standard time, when clocks advance directly to 3:00 a.m.
United States, autumn 2026. 1 November is ordinal day 305, so it is 304 days after 1 January, and 304 mod 7 = 3 (7 × 43 = 301). Thursday plus three is again Sunday, so 1 November 2026 is itself the first Sunday: D = 1 November 2026, at 2:00 a.m. local daylight time, when clocks return to 1:00 a.m. and the hour from 01:00 to 02:00 occurs twice.
Days on summer time. 8 March is ordinal day 31 + 28 + 8 = 67. 1 November is ordinal day 305. The difference is 305 − 67 = 238 days on daylight saving, and the remaining 133 days of the cycle to the next spring transition on 14 March 2027 are on standard time.
European Union, 2026. March 2026 has 31 days and 1 March is a Sunday, so the Sundays are 1, 8, 15, 22 and 29 — the last is 29 March. October 2026 has 31 days; 1 October is ordinal day 274, which is 273 days after 1 January, and 273 mod 7 = 0, so 1 October is a Thursday. The Sundays are then 4, 11, 18 and 25, giving 25 October. Both transitions occur at 01:00 UTC, which is 02:00 and 03:00 respectively on a central European clock. The summer runs from ordinal 88 to ordinal 298, which is 210 days.
What the result means for scheduling and for logs
Two clock anomalies follow from every transition, and both cause real failures. At the start of daylight saving an hour of local time does not exist: in the US, 02:00:00 to 02:59:59 local is skipped entirely on the March Sunday. A cron job set for 02:30 will not run that day, and a timestamp of 02:30 on that date is invalid. At the end an hour repeats: 01:00 to 01:59 occurs twice, once on daylight time and once on standard time. A log ordered by local timestamp will appear to go backwards, and a job scheduled for 01:30 may run twice.
The defence is to schedule anything sensitive in UTC, or outside the 00:00–04:00 local window. Financial reconciliation, backup rotation and billing cut-offs are the usual casualties. If you must store local times, store the UTC offset alongside them; a bare local timestamp in the repeated hour is genuinely ambiguous and no amount of later processing can resolve it.
For meeting schedules across regions, note that the US and EU transition dates do not coincide. In 2026 the US springs forward on 8 March and the EU on 29 March, so for three weeks the usual five-hour gap between New York and London becomes four. The same happens for a week in autumn, when the EU falls back on 25 October and the US on 1 November. Those two windows are when recurring cross-Atlantic meetings drift, and the time zone converter is the tool to check a specific date against.
Roughly a third of the world's countries observe daylight saving, almost all of them in the middle latitudes. Equatorial countries do not, because day length barely varies there — a point the daylight hours calculator makes concrete. Several jurisdictions have abandoned it recently, and within observing countries there are exceptions: Arizona and Hawaii in the United States, Queensland and Western Australia in Australia, Saskatchewan in Canada.
The statutory rules by region
| Region | Begins | Ends | Instant specified as | Governing instrument |
|---|---|---|---|---|
| United States, Canada | 2nd Sunday in March | 1st Sunday in November | 02:00 local | Energy Policy Act 2005 |
| European Union | Last Sunday in March | Last Sunday in October | 01:00 UTC | Directive 2000/84/EC |
| United Kingdom | Last Sunday in March | Last Sunday in October | 01:00 UTC | Summer Time Order 2002 |
| Australia (NSW, VIC, SA, TAS, ACT) | 1st Sunday in October | 1st Sunday in April | 02:00 local standard | State legislation |
| New Zealand | Last Sunday in September | 1st Sunday in April | 02:00 local standard | Time Act 1974 order |
| Arizona, Hawaii, Queensland, Western Australia | No daylight saving observed | — | Local exemption | |
Chile, Iran, Israel, Paraguay and several others follow rules of their own that change from time to time; check the IANA time zone database for those rather than assuming a pattern.
Assumptions and limits of this calculator
- It applies today's rule to every year you enter. Before 2007 the United States started daylight saving on the first Sunday in April; the calculator flags this when you enter an earlier year, but it still shows the modern rule.
- It covers five rule sets, not every jurisdiction. Chile, Iran, Lebanon, Mexico and others use different or recently changed rules. For those, read the IANA time zone database rather than a formula.
- It assumes the rule stays in force. The European Parliament voted in 2019 in favour of ending seasonal clock changes, and legislation to do so has not been adopted; several US states have passed permanent-daylight-time laws that require federal action to take effect.
- It reports the clock time, not the UTC instant, for the American and Australasian rules. Those rules are local, so different time zones inside one country change at different absolute moments.
- It does not handle half-hour and 45-minute zones' local nuances. Lord Howe Island shifts by 30 minutes rather than an hour, which no general rule captures.
Where the rules came from, and what to use alongside
Daylight saving was introduced widely during the First World War as a fuel-conservation measure, Germany and Austria-Hungary first in April 1916, with Britain and others following within weeks. The United States standardised the practice in the Uniform Time Act of 1966, which let states opt out entirely but not adopt their own dates. The 2005 extension added about four weeks — three in March, one in November — on an energy-saving rationale whose measured effect has been small and contested in the literature since.
For anything operational, the authoritative machine-readable source is the IANA time zone database (also called tz or zoneinfo), which encodes every historical and current rule for every zone and is updated several times a year as governments change their minds. Operating systems, programming languages and databases all ship a copy. A formula like the one above is right for the current rule in the five regions listed and is the wrong tool for a historical timestamp from 1994 or a Chilean date.
Once you have the transition dates, the other tools follow: use the time zone converter to translate a specific meeting time across the mismatch weeks, the time duration calculator to work out a shift length that spans a transition, and the Unix timestamp converter when you need the unambiguous epoch value that no clock change can disturb. The day of the week calculator is the underlying primitive: every rule here is just a weekday question.