To Foot

Summer Time.

(U.K., E.U., and other)

General Summer Time

The purpose of Summer Time is to reduce wasted morning daylight by making the active period of the day (which is largely fixed by civil time) earlier in real time; therefore, clocks are put ahead for the Summer. It is equivalent to a temporary change to a different Time Zone (eastward, usually to the next zone). It yields modest energy saving.

The Summer Time rules for most places (including UK, EU; & NA) imply that the choice of each transition date from the seven possibilities is in one-to-one correspondence with the day-of-week of New Year's Eve (and of Christmas Day) at the end of that year.

In (almost) any place that has Summer Time, an hour of Civil Time is omitted in local Spring^* and an hour is repeated in local Autumn^*. German Law has said that the duplicated hour shall be referred to as 2A & 2B.

Double Summer Time (with "Single Winter Time") is equivalent to a year-round eastward change of Time Zone; the clocks are set two hours ahead of Standard Time in Summer, and one hour in Winter.

* Seasons are approximate.

British Summer Time

In this page, statements about the European Union Summer Time rules are largely derived from the English version (which uses GMT; others do not), and GMT often means GMT/UTC.

In the EU, Summer Time is used from the last Sunday in March until the last Sunday in October. The changes occur, in all member countries, at 1 a.m. GMT/UTC (legally, UK time is GMT-based). UTC differs from GMT by less than a second. As the changes both occur at the same UTC, Summer Time and Winter Time last for an exact number of weeks.

Thus, in the UK and the adjacent Crown Dependencies, in Spring at the ultimate approach to 1 a.m. GMT the civil time jumps forward to exactly 2 a.m. BST and in Autumn at exactly 2 a.m. BST the civil time jumps back by an hour to just after 1 a.m. GMT.

That is in agreement with The Summer Time Order 2002, dated 12th February 2002 and in force from 11th March 2002. The Order implements the Ninth Directive 2000/84/EC of the European Parliament; it also repeals previous powers for variation.

The EU directives (in English) specify GMT, and imply that Summer Time includes the instant of 1 a.m. GMT at each end; likewise for the Summer Time Order. But it is computationally more convenient to consider the clocks to change immediately before arrival at that instant, so that 1 a.m. GMT itself is in the new time.

For the UK and the rest of the EU, October is thus the longest month of the year.

EU Summer Time lasts for 30 weeks if starting on March 29-31 (and finishing four days earlier in October), and 31 weeks for March 25-28 (three days later). It always ends in ISO Week 43 (yyyy-W43-7), except in years with Sunday February 29th (yyyy-W44-7); 2004, 2032, ... when it ends on October 31st.

In the UK, correct standard clocks show GMT in the Winter, and show BST, which is one hour later than GMT, in the Summer. In Ireland and Portugal, civil clocks show the same time as in the UK. Most other EU clocks show one hour later than UK ones, to as far east (roughly) as Warsaw and Budapest, but not to Athens and Helsinki, where clocks show two hours later than UK ones.

The change, by EU rules, can occur on Easter Sunday; previous UK policy was to avoid that. If Easter is very early, the change may be a week after Easter (2008, 2160, 2285, ...). Gregorian Easter dates are listed here.

Occasionally, a UK clock change has been on a day other than Sunday - Autumn 1917-1921 (from BST), Spring 1945 (to BDST).

Meanings of BST

BST is used for British Summer Time, British Standard Time, Bangkok Standard Time, Brazil Standard Time, and Bering Summer Time. And, according to Wikipedia BST, Bangladesh Standard Time, Burma Standard Time, Bissau Standard Time, Burkina Faso Standard Time.

British Double Summer Time

Double Summer Time (BDST/DBST) has been used in the UK, during 1941-45 and 1947. Between Summer 1940 and Summer 1945, civil time in the Winter was an hour ahead of GMT. BDST is no longer allowed. See Myers's page and NPL's archive, via Links.

British Standard Time

From 1968-10-27 to 1971-10-31, we had British Standard Time - an hour ahead of GMT, all year round. See Myers and NPL, via Links.

Therefore, the base of UNIX/javascript time, 1970-01-01 00:00:00 GMT, was at 1 a.m. UK civil time.

No Early Change by 1972 Rules

The 1972 rules did not allow a change on Easter Sunday.

Easter Sunday can only be on March 22nd - April 25th. The 3rd Saturday of March can only be on March 15th - March 21st. Thus the set-back only applied for Easter Sunday on March 22nd, which is rare. The only March 22nd Easter Sundays in 1583-2500 are :-
  1598 1693 1761 1818 2285 2353 2437
Thus, even if that rule had continued policy existing earlier, it can never have had effect.

Single Double Summer Time ?

(S.Tel, 2006-03-19 p.2) A Bill in the Lords would, for a three year trial, advance British Civil Time for an hour, from 2006-10-29; SDST = winter GMT+1, summer GMT+2, as in 1940-45.

October 2006 : the proposal is again raised, but change does not seem likely.

December 2006 : Tim Yeo MP, Private Member's Bill, in Commons, for three-year European Time experiment, Scotland not necessarily participating.

British Clock Change Times

I know of no authoritative full list of the times of the changes between Winter and Summer Time; but it may be possible to generate one by looking in past copies of newspapers.

These are deduced from Myers on the assumption that, if unspecified, legislation took effect promptly.

Before 1981, all changes were between 02:00 and 03:00 clock time. Since then, all changes have been between 01:00 and 02:00 clock time.

Before 1981, changes to/from GMT were at 2 a.m. GMT; and at 1 a.m. GMT thereafter. Changes to/from BDST were at 1 a.m. GMT.

There was no two-hour change (between GMT and BDST).

From 1916, Britain has always had advanced time in Summer, and has always used GMT+1 in late Spring and early Autumn.

Proposed Changes

It has been suggested (House of Lords, 2005-04-02, "British Summer Time") that the European Union should alter the start of Summer Time from the last Sunday in March to the third Sunday so that clocks are not put forward on Easter Sunday.

Authorities

The following quotations must be read in full, noting the dates received; later parts can supersede earlier ones.

   1998: March 29 - October 25
   1999: March 28 - October 31
   2000: March 26 - October 29
   2001: March 25 - October 28

European Time Zones

The Azores (and South-East Greenland) are in the GMT-1 Time Zone.

Portugal, Ireland, and the UK are in the GMT Time Zone (as are the Isle of Man, the Channel Islands; the Faeroes; the Canaries, Iceland).

Otherwise, the mainland of Western Europe, i.e. Norway, Denmark, France, Spain, and eastwards, is in the GMT+1 zone (as is former Yugoslavia and Albania).

Finland, Estonia, Latvia, Lithuania, Kaliningrad, Belarus, Ukraine, Moldova, Romania, Bulgaria, Turkey, Greece and Cyprus are in the GMT+2 zone. Moscow is in GMT+3; European Russia also uses GMT+4 and probably some GMT+5.

See also in Time Zones.

Non-British Summer Time

Europe

From Spring 1996, or maybe Autumn 1995, British Summer Time has been governed by EU rules. Trivial ambiguities (GMT or UTC) apart, the whole of the European EU, with a number of nearby territories, changes between Winter Time and Summer Time simultaneously.

R.H. van Gent's Herinvoering van de zomertijd has details, including a list of Directives.

EU Rules

EU: "For geographical reasons, the common summer-time arrangements should not apply to the overseas territories of the Member States". That is odd : it should read "need not".

The EU intends its present rules to be permanent, with the changes at 0100h UTC (GMT according to the English version of their Web site) on the last Sundays of March and October, for every Member State.

On present EU rules, Summer Time lasts for either 30 or 31 weeks. The Day-of-October number is either three bigger or four smaller than that of the Day-of-March. The mean difference is zero, so that, on average, Summer Time lasts seven calendar months (214 days), whereas Winter Time averages five calendar months (151¼ days) (ignoring missing Leap Years).

For the composition of the EU, see my Europe and Euro. I believe that the countries which joined in May 2004 use the same Summer Time rules as earlier members, and that future joiners will do likewise.

Gibraltar, like Spain, uses Central European Time.

Myers includes further information.

Start of Present EU Rules

I've read :- Some European countries changed dates in response to a European Union directive to standardize daylight time beginning in 1996.

It could be useful to know in which year each European country finally started using the current EU change date rules (i.e. the Last Sundays of March and October at 01:00 GMT/UTC), whether fortuitously or by intent. RSVP. Those given here are from various sources, and should be checked.

In the UK, changes have been in the last third of October from 1961, and in the last week of March from 1981.

"Hours Offset" is the number of hours by which local time, in Winter, is greater than GMT/UTC; add one hour for Summer.

I do not know to what extent the Continental countries had, before the present rules, common rules. It seems that the present pan-EU rules were standardised from 1981 or before, except as shown.

The wording of EU directives implies that EU Summer Time rules apply to all Member States from the moment of joining.

NPL says that all EU states used the present rules from 1995-10-22.

Non-EU Locations

The Isle of Man and the Channel Islands match the UK, and therefore the EU; likewise, Ceuta and Melilla match Spain and so the EU.

Andorra, Norway, Switzerland, and Turkey match the EU, I gather; as does most or all of the remainder of Eastern Europe. Liechtenstein, Monaco, San Marino, and Vatican City presumably match the surrounding or adjacent countries.

Iceland uses only GMT/UTC.

For Russia and other fUSSR, see below.

North America

USA

From 1987 to 2006, the USA started Daylight Saving Time on the first Sunday of April and finished it on the last Sunday of October (previously to that, the last Sundays of both months). The clock changes were on reaching 2 am local time.

From 2007, DST is from the second Sunday of March until the first Sunday in November; the clock changes remain on reaching 2 am local time - Spring: 02:00 to 03:00 and Autumn: 02:00 to 01:00. As the changes both occur on reaching same local clock time, Summer Time and Winter Time each differ by an hour from an exact number of weeks.

Federal law makes the use of DST a local option, but the rules are otherwise nation-wide. DST does not apply in Hawaii and much of Arizona; and not in non-contiguous territories [Puerto Rico, the Virgin Islands, American Samoa, Guam and the Northern Marianas].

Check with local sources, such as USNO.

Some US software implemented the 1987-2006 Spring change date as April 8th in years when it should have been April 1st.

American Alterations

See also Risks.

Double Summer Time

There was (May 2001) a proposal (HR 704) to consider Double Summer Time for the Western USA.

Changing Exclusions

Before 2006, southern Indiana did not use DST.

Federal DST Date Rule Change 2006/07

Found by search of a US Congress site for daylight saving :-

The Bill was signed into law by President G.W.Bush on 2005-08-08 (Public Law 109-58). Possible subsection (c) Report (1.42MB PDF)? There has been a recent (Nov 2008) Report.

From 2007, DST thus starts three or four weeks earlier than before and ends one week later than before.

Rules using the N'th (including the 0'th, but not the Last) Sundays of March and November make Summer Time of constant length - 34 weeks for the new US dates. The length of Winter Time will vary.

"Spring forward, Fall back" has become inaccurate. Easter is now always in DST. Some Election Days are now in DST - 2010 2032 2038 2060 2066 2088 ... .

Note that the change formally took effect on 2007-03-01, presumed at 00:00:00 local time. That is in principle important in JavaScript, which is required to use only the current rules for all dates.

Canada

Canada (excluding most of Saskatchewan and some of Québec, which have no DST, and with local variations) changes on the same dates, but not everywhere at the same times, as the USA does. At NRC, Canada, Time Zones & Daylight Saving Time, When does daylight saving time start and end gives details; mostly when the clocks reach 02:00, partly when the clocks reach 00:01, Manitoba at 02:00 Winter Time. Provinces choose their own rules.

Canadian Changes

Following the USA, all of Canada that uses DST has changed to the new dates.

Bahamas, Bermuda

Both changing as for the USA. Also maybe Turks and Caicos Islands.

Others

Mexico generally (from 1996) uses Summer Time; part has followed the adjacent USA (1987-2006) rules, part has had shorter Summer Time. Mexico retains the US 1987-2006 rules.

Cuba has Summer Time.

Elsewhere

Summer Time is not needed near the Equator.

The examples that follow are merely illustrative; they have largely been taken from seemingly-reliable sources rather that from authorities, and they may have changed since.

ISS

The International Space Station uses Greenwich Time.

Northern Hemisphere

Russia changes on the EU dates; the last Sunday of March and the last Sunday of October. I believe that all parts change at the same local time. The non-European parts of the former USSR seem to match Russia.

Russian Space Mission Control keeps Moscow Standard Time (GMT+3) all year round.

In Israel, Summer Time is now from 02:00 on the Friday preceding April 2nd to 02:00 on the Sunday between Rosh Hashana and Yom Kippur. That policy cannot be sustained indefinitely, as average Hebrew & Gregorian year-lengths differ.

Palestine varies, ending Summer later.

Egypt, South West Africa, and Tunisia have Summer Time.

Japan and Korea (GMT+9) do not change; and all China currently stays on GMT+8.

Southern Hemisphere

Naturally, the Southern Hemisphere changes oppositely to the Northern.

Up to winter 2007, New Zealand changed, at 02:00h local standard time, on the first Sunday in October and the third Sunday in March. Now the changes are on the last Sunday in September and the first Sunday of April.

In Australia: NSW, ACT, Vic, & SA apparently have changed, at 02:00h local winter time, on the last Sunday of October and the last Sunday in March. Que, NT, and WA do not change (but WA has a 3-year DST trial starting Dec 2006, according to MS). Tasmania changed at that time on the first Sunday of October and last Sunday of March. Dates need checking. Other territories can vary.

2008-03-28 : From this year on, daylight saving in NSW, Victoria, ACT, Tasmania and South Australia will end a week later than usual on the first Sunday in April and, with the exception of Tasmania, recommence three weeks earlier on the first Sunday in October. By Asher Moses.

See also, via Lawlink, Daylight Saving in New South Wales.

Chile, Paraguay, Uruguay, and parts of Brazil have Summer Time. The Falklands have Summer Time. Antarctica varies.

Discrepant Years

According to Risks Digest 20.94, in much of Australia Summer Time started abnormally early in 2000 (on last Sun in Aug, instead of in Oct), to bring the Olympic Games in Sydney into the Summer. However, much of Australia has no Summer Time.

In Autumn 2006, Australia extended Summer Time by a week, to April 2nd, for the Commonwealth Games.

Chile delayed a change date in 1987 and 1990. The USA had special changes in 1974/5. Etc.

No Summer Time

There is no Summer Time in most of South America, almost all of Africa, Arabia, the Indian sub-continent, and the Far East; except that I have read that Pakistan will have it, to match its friends nearby.

Permanent GMT

Some countries, including Iceland and Ghana, keep GMT all year round.

A Proposal for Rationalisation

Clearly the Southern Hemisphere will never put its clocks forward and back on the same dates as the Northern; and, as it seems to be generally agreed that Summer Time should be longer than Winter Time (starting near the Spring Equinox but finishing over a month later than the Autumn Equinox), they will not change in the opposite direction on the same dates either. There is therefore an essential disagreement between those who have Summer Time in July, those who do not have it, and those who have it in January. The Lines of Disagreement follow, very approximately, the Tropics of Capricorn and Cancer, with some regions being anomalous.

In the Northern Hemisphere, the vast majority of places which change their clocks have chosen to do so on the last Sundays of March and October. In Europe, all but the Far East do it at 01:00 UTC, and in Asia virtually all outside the Middle East apparently change clocks on the same dates but at a fixed local time.

The only^† Northern country which has chosen to use different dates is the USA.

The difficulty that Americans have with changing differences between their local time and that elsewhere is therefore largely of their own creation. They should use the same dates as the vast majority of Northern Hemisphere clock-changing countries and a substantial majority of Northern Hemisphere clock-changing people. Then they would only have change-date problems when dealing with the Deep South.

† Canada made no choice; there, the matter is left to the Provinces. The Provinces all made the same choice of date, but they did not make it simultaneously. Mexican rules also depend on location. The rest of clock-changing North America is in practice a minor detail.

Risks in Alterations to Rules

Parts of this refer specifically to the US DST change in 2007, or use it as an example.

General

Changes in the Rules which govern the date and/or time at which the civil clock offset is adjusted will naturally cause general confusion, especially to the public and the media. For example, the 2007 US change has been described as "starting three weeks earlier" - but in about three years out of seven, it will be four weeks, not three (the end is always one week later).

General date/time risks are in my Critical and Significant Dates; general risks are in Risks Digest.

Any change should have interesting effects; it was the first time for the USA since the proliferation of PCs began. Risks Digest references : issues 23.94, 23.95, 23.96 ... . Problems reported as occurred : Risks Digest references : issues 24.59 ...??? .

Some Changes

USA, 2007-03-01; Canada, matching.

New Zealand changed in winter 2007 - Risks Digest 28.86 refers.

There was a suggestion that Russia might drop Summer Time in 2009. Turkey may move to permanent GMT+2.5 in 2011.

Venezuela changed time zone from GMT-4 to GMT-4.5 in 2007.

Hardware - Computer and Other

In the PC RTC ("CMOS"), Register B bit 0 is "Daylight Savings Enable", according to CMOS.LST in RBIL, with the US 1987-2006 rules. I don't know whether the feature is ever used.

Such chips, or corresponding logic, may be employed in other devices - DVDs, VCRs, white goods, etc. But devices that receive locally-broadcast signals do not need them.

System Software

System software needed to be updated; in some cases by merely changing parameter files, in others by changing actual code. If the change was omitted, a computer getting time as GMT/UTC from a server would still be correctly set for GMT, but would show the wrong local time during the annual discrepant intervals - and if the visible local time is then manually adjusted, the UTC will then be wrong.

The new US rules should not have been loaded until a week after the end of DST 2006, unless the code is year-sensitive. When the new US rules have been loaded, DST for 1987-2006 will be wrongly calculated, unless the code is year-sensitive.

It may be necessary to update both the rules currently in active use and those in the general database, if the chosen location may later be changed.

DOS-type file datestamps will not be affected; but some American Windows-only ones with values before 2007 held as 100ns units from Gregorian 1601-01-01.0 GMT may appear to change by an hour.

There is a Windows XP update, and an Apple one : see in External Links.

Wikipedia, "Daylight saving time", Microsoft Windows systems says that Vista introduces two sets of rules per location.

Networked machines are normally synchronised by receiving UTC. So, if the old rules are still active during the discrepant period, the best move may well be for that period to set the alleged location one (one-hour) time zone to the West.

Specific Applications

Software such as GAWK, ported from UNIX to DOS/Windows, which probably originally implemented the UNIX TZ variable in full using a code library, may have had the US 1987-2006 DST rules hard-coded.

From 2007, the transatlantic civil time difference differs from normal for four or five weeks in each year instead of the previous one week, affecting airline timetabling.

Where appointments are stored in UTC (e.g. MS Outlook), those made in civil time before the update is applied for times affected by the update may be in error by an hour; analogous to the problems seen by those who travel across Time Zones.

JavaScript

JavaScript is required by ECMA-262 3rd Edn to use the current Summer Time rules for all years; some browsers on some operating systems, including UNIX types, do not comply.

Test case (assumes that the absence of Summer Time data for a year is taken as meaning no Summer Time) :-

Better, though, to test, in a place where rules have changed, a suitable day of the year :-

Americans could test late March of 2006 and 2007 for Offset; those should never differ, but should change at the start of 2007.

JavaScript code modified for your actual locality can be tested there.

For more on JavaScript and Summer Time, see OS Time Offset Settings

Safe Data Processing

For records when the true time may really matter, one could record both the local time and the offset from GMT/UTC. But it is more logical to record GMT/UTC and the offset to local time for each event.

For the difference between UTC and GMT, see in Leap Seconds.

Where the date/time functions of a computer language are Summer-Time-aware, peculiar things may happen if date/time arithmetic is done at or across offset changes. That can apply anywhere; but note that in the Azores (and South-East Greenland) clocks should change to/from midnight. Part of Argentina also changes at midnight.

Notes

Nature of Clock Changes

As far as I recall, the change is currently always (with one exception) a matter of altering the clocks on a weekend night, ahead by one hour for Summer Time and back for Winter Time. A weekend is not everywhere Saturday and Sunday, however.

Remember that Southern Hemisphere Summer Time spans the New Year.

Anything other than optional one-hour changes, one per year per direction, would seem likely to cause considerable confusion in present-day computing. But see Markus Kuhn, "Unifying Atomic Time and the post-Gregorian calendar corrections".

Time of Clock Changes

Some places, including UK/EU, change at the same GMT in both Summer and Autumn; their Summer Time is a multiple of 24 hours long. Others, including the USA, change the clocks when they first indicate a given local time in each case; their Summer Time is not a multiple of 24 hours long.

The same rule may cover areas in different Time Zones. In some cases, including UK/EU, the change is simultaneous in all Zones; on others, including the USA, the change is at a given local time.

Greenland follows EU rules (being quasi-Danish), changing at Sunday 01:00 UTC, which for most of them is still Saturday LCT.

The time-of-change rules for a given region may occasionally be altered.

Ambiguous & Impossible Hours

For a given locality, a given real instant, say in GMT, can always be transformed into a local date & time. But, where Summer Time is used, the converse is not always true.

Let X stand for "about 24×365.25=8766". Then, given a random real instant, there are two chances in X that the conversion to local date & time cannot unambiguously be reversed, unless the difference from GMT is effectively saved and used. Given a random time T in 00:00 ≤ T < 24:00 on a random day of the year, there is one chance in X of it being impossible, and one chance in X of it being ambiguous.

Who uses GMT/UTC?

GMT and/or UTC are often used by larger computer systems; UNIX maintains GMT.

Outside computing and other specialisms, as far as I can see :-

• The British Isles, Iceland, part of Greenland, Portugal, West Africa (from Morocco to Togo)^* and nearby Atlantic islands are in the "12" time zone, and use GMT in the Winter at least (does West Africa need Summer Time? no).
• The Azores and South-East Greenland are in the "11" zone, and so are on GMT in Summer Time; they use EU rules (most of Greenland is in "9", some is in "8")
• Some South Atlantic islands are in "10"; and Double Summer Time would bring them to GMT.
• ISS.

^* St Helena, Burkina Faso, Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, Morocco, Sao Tome, Senegal, Togo, Western Sahara, according to GreenwichMeanTime.

Logical Schemes

To be rational, Summer Time ought to be from the Sunday nearest to the nominal Spring Equinox (which in the Northern Hemisphere is March 21st, implying the Sunday in 18th-24th) to that nearest to the Autumn one. Over any reasonably substantial contiguous region, the change should occur simultaneously, at 03:00 average local winter time; except that Russia is too extended in longitude for this to be practical. But the world is not rational.

The Equinoxes could be approximated as the Third Sundays of March and September; or, allowing for the Autumn Equinox generally being warmer than the Spring one, the change could be at some fixed number of weeks after that or some specified Sunday of April and October.

Then both North and South of the Equator could change, albeit in opposite directions, on the same local dates.

Alternatively, there being N = 52 or 53 ISO 8601 weeks per year number, the change could be on the last day of Weeks 13 & 39, or 13 & N-13; or some given equal number of weeks later than those.

Since clocks are advanced for the majority of the year and retarded for the minority of the year, it would seem better to have Standard Time in Summer, with Winter Time an hour behind.

Multiple Changes

A machine with multiple operating systems may have the change applied by each in turn. Probably this only applies to machines, like PCs, in which the central clock keeps local civil time.

Brief History of Summer Time

Putting Local Time ahead of Solar Time during the Summer is commonly said to have first been suggested (apparently frivolously) in an essay by Benjamin Franklin (Paris, 1784). But that essay suggests only early rising and retiring, not altering the clocks.

British

Summer Time was advocated in a pamphlet ( text of 1st edition, July 1907 (+), PDF of 19th edition, March 1914) by William Willett^* (1856-1915) (a builder; of Chislehurst, Bromley, London; Blue Plaque); he had the idea in 1905, while riding on a nearby common. Its introduction was considered in the Commons⁺ in 1908/9, but rejected.

^* : Some sites say that he was an MP; but the House of Commons Information Office states that he was not.
⁺ : The Daylight Saving Bill was introduced by Mr (later Sir) Robert Pearce, Liberal, Member for Staffordshire, Leek.

Sir Ernest Shackleton, on Endurance ice-bound in the Weddell Sea, advanced the expedition's time by one hour on Sunday 1915-09-26 (see his book: South, p.56, ISBN 1-86105-279-0).

In the UK, British Summer Time was first introduced on Sunday 1916-05-21 at 02:00 GMT. British Double Summer Time (and advanced time in Winter) was used during WWII. In every year from 1916, UK civil time has been ahead of GMT in Summer.

Elsewhere

Local

Various Canadian municipalities seem to have implemented Summer Time in about 1907-16. For example, Moose Jaw, Saskatchewan, introduced Municipal DST on Saturday 1912-06-01, until "the end of the summer".

National

Germany and Austria introduced Summer Time on Sunday 1916-04-30 at 23:00.

Canada adopted Daylight Saving Time generally in 1916.

In the USA, Daylight Saving Time was first used in 1918/19; reintroduced nationally during WWII; and rendered more orderly by the Uniform Time Act of 1966 (revised 1972, 1986, 2005).

At some places and times, time has been advanced (as Summer) all year round; and other changes have been made for particular circumstances such as energy crises.

Web references include, at webexhibits.org, c.html, i.html, and others.

Determination of Change Dates

This section generally assumes that the present or recent (as at 2007) rules hold; be cautious when dealing with dates before the recent past.

On This Web Site

My DOS program MJD_DATE will calculate EU/UK Summer Time dates. My DOS program NOWMINUS can correct EU civil time to GMT, for the DOS Environment. These have the present rules built in, not relying on the OS.

In a Win32-compatible system, using OS rules, my Delphi program STD_TIME prepares the GMT, time_t, and local CMJD for setting into an environment, readable by NOWMINUS.

Web page JavaScript Date and Time 5 : Date and Time Elsewhere at present has Civil Time calculators, converting the reader's computer's time to London, New York, Beijing, and Anywhere Time.

The state of Summer Time can be deduced from the offsets from GMT for January, July and the current date. Web page JavaScript Date and Time 2 : Demonstrations has JavaScript code for determining the state of local Summer Time from the OS settings. Web page JavaScript Calendars and Clocks gives, for the year displayed, the propers UK/EU change date/times, and also the local dates given by the OS.

General Calculation

For rules as those of EU & NA (changing on Nth or last Sunday of the month) : Determine the day-of-week of the last possible date - e.g. Mar 14 / Mar 31 / Apr 07, Oct 31 - then find the modulo-7 difference between that and Sunday, which will be in 0-6, and go back that many days.

Simple Formulae

The following simple formulae are derived from a Web page at webexhibits.org :-

EU Summer Time, current (as at 2007) rules :-
   Begin    March (31 - (5*y div 4 + 4) mod 7) at 01:00 UTC
   End    October (31 - (5*y div 4 + 1) mod 7) at 01:00 UTC

US DST, rules 1987-2006 :-
   Begin    April (07 - (5*y div 4 + 4) mod 7) at 02:00 local
   End    October (31 - (5*y div 4 + 1) mod 7) at 02:00 local

US DST, rules from 2007 :-
   Begin    March (14 - (5*y div 4 + 1) mod 7) at 02:00 local
   End   November (07 - (5*y div 4 + 1) mod 7) at 02:00 local
				     ^

They implement the stated rules, and are valid for years that those rules apply within 1900-2099. For other centades, the constant term (marked ^) changes, as shown in R.H. van Gent's Zomertijd calculator.

A similar rule applies for other fixed dates, such as Christmas Day.

Calculating with them :-

Lengths of Winter and Summer Time

This uses UTC dates for simplicity and efficiency. Transitions are calculated by DateFor from their last possible date by moving back 0-6 days to the right day-of-week.

The TZ Variable

The Summer Time rules are often given by a DOS/UNIX Environment Variable TZ - see Time Zone. Almost all places use M-type rules; month, week-of-month, day-of-week. See Generate a User TZ String.

From OS Settings

If the offset from GMT is known for the current date and for January & July, then the state of Summer Time can be deduced. Web page JavaScript Date and Time 2 : Demonstrations has JavaScript code for determining the state of local Summer Time from the OS settings.

Microsoft Windows

It appears that Microsoft used to put the UK/EU transition time at 0200h UTC, rather than at 0100h GMT (in, at least, MSIE 4 in 1st release of Win98; and also in a version of NT). My JavaScript Calendars and Clocks will show you when your browser thinks your system changes. For all years, my MSIE 4 JavaScript correctly used what it thought were the present rules.

My Windows XP sp2 got it right.

To convert between local time and GMT/UTC, see "TIME_ZONE_INFORMATION" etc., in the online Win32 Programmer's Reference help. It says that GetTimeZoneInformation is a function whose return value indicates TIME_ZONE_ID_UNKNOWN / TIME_ZONE_ID_UNKNOWN / TIME_ZONE_ID_DAYLIGHT ; lpTimeZoneInformation points to a TIME_ZONE_INFORMATION structure which is given the details of changes. The information is held in the Registry.

My copy of the Help in Win98 1st Edn, appeared nearly correct (see tz-check.pas). The times were reported as 0200h & 0300h; but the latter seems not GMT/UTC.

In directory programs, tz-check.(pas,exe) is a short Win32 console mode Delphi 3 program demonstrating these things.

I have heard that TZEDIT, on the Windows CD (NT: resource kit), can alter these settings.

Evidently Windows has, somewhere, the "current" rules for "all" locations; "Date/Time Properties, Time Zone" can pick from these. For the UK, pick Dublin..., not Casablanca...; and tick the adjustment box. My Windows 98 may have used c:\windows\system.nav for this.

TZ-in-NT.txt, which discusses Windows NT and Summer Time in relation to the UK, is part of an E-mail dated 2002-10-23. It is reproduced with the permission of the author, Chris Freestone.

Dual-boot systems may dual-change, when Summer Time starts or stops.

Beware possible effects on apparent Windows file timestamps; Risks Digest 22.35.

Oddities

All of Great Britain (probably all of the British Isles, and the Channel Islands too?) keeps London time, with, I have Clive Feather, 23 Aug 2004, news:uk.transport.london, Message-ID: RJBADr37SlKBFw8A@romana.davros.org read, a small exception, variously described : ROT-13 :-

• Gur Punaary Ghaary Grezvany hfrf Serapu Gvzr.
• Gur Punaary Ghaary Pbaprffvba Nern hfrf Serapu Gvzr.
• Nyy gur pbzchgre flfgrzf sbe gur Punaary Ghaary ba obgu fvqrf bs gur punaary hfr Serapu pybpx gvzr.

Lord Howe Island (Australia) has a half-hour time shift.

BRITISH SUMMER TIME DATES

The dates themselves, originally obtained from various sources, are all in agreement with the NPL archive linked below.

BST is GMT+1; UK civil time is otherwise GMT.

Actual Dates, up to 2007

Dates before 1968

See NPL site.

Dates 1968-1971

  1968  UK Feb 18 - Oct 27, then British Standard Time, also GMT+1
  1969  UK Brit. Std. Time
  1970  UK Brit. Std. Time
  1971  UK BrStdT - Oct 31

Dates by 1972 UK Rules, 1972-1980

The Rules were effectively "Day after Third Saturday in March to day after Fourth Saturday in October" : March 16..22 to October 23..29.

  1972  UK Mar 19 - Oct 29
  1973  UK Mar 18 - Oct 28
  1974  UK Mar 17 - Oct 27
  1975  UK Mar 16 - Oct 26
  1976  UK Mar 21 - Oct 24
  1977  UK Mar 20 - Oct 23
  1978  UK Mar 19 - Oct 29
  1979  UK Mar 18 - Oct 28
  1980  UK Mar 16 - Oct 26

Dates by 1981 UK Rules, 1981-1994

The Rules were "Last Sunday in March to day after Fourth Saturday in October" : March 25..31 to October 23..29. The start date agrees with the current rule.

Where marked * below, the current UK/EU Rules would end the period a week later.

  1981  UK Mar 29 - Oct 25
  1982  UK Mar 28 - Oct 24 *
  1983  UK Mar 27 - Oct 23 *
  1984  UK Mar 25 - Oct 28
  1985  UK Mar 31 - Oct 27
  1986  UK Mar 30 - Oct 26
  1987  UK Mar 29 - Oct 25
  1988  UK Mar 27 - Oct 23 *
  1989  UK Mar 26 - Oct 29
  1990  UK Mar 25 - Oct 28
  1991  UK Mar 31 - Oct 27
  1992  UK Mar 29 - Oct 25
  1993  UK Mar 28 - Oct 24 *
  1994  UK Mar 27 - Oct 23 *

Dates in 1995

Anomalous end date.

  1995  UK Mar 26 - Oct 22 *   (Ireland matched the UK)

Dates matching current UK/EU Rules, 1996-2007

The Rules are "Last Sunday in March to Last Sunday in October" : March 25..31 to October 25..31.

UK Summer Time, by the above EU Rules and by Directives, etc. :-

  1996  :  Mar 31 - Oct 27
  1997  :  Mar 30 - Oct 26
  1998  :  Mar 29 - Oct 25
  1999  :  Mar 28 - Oct 31
  2000  :  Mar 26 - Oct 29
  2001  :  Mar 25 - Oct 28
  2002  :  Mar 31 - Oct 27
  2003  :  Mar 30 - Oct 26
  2004  :  Mar 28 - Oct 31
  2005  :  Mar 27 - Oct 30
  2006  :  Mar 26 - Oct 29   SDST would omit the Oct 2006 clock change
  2007  :  Mar 25 - Oct 28

Dates for 2002-7 have been seen on the EU and/or HMG Web Sites.

Extrapolated Dates, from 2008

Naturally, with constant Rules disregarding Easter, the date pattern repeats every 28 years from 1998 to 2099, and (shifted) from 2100 to 2199, etc.

Dates for 2008-11 have been seen on the NPL Web Site.

  2008  :  Mar 30 - Oct 26 - 2036, 2064, 2092; 2104, ....
  2009  :  Mar 29 - Oct 25 - 2037, 2065, 2093; 2105, ....
  2010  :  Mar 28 - Oct 31 - 2038, ....
  2011  :  Mar 27 - Oct 30 - ....
  2012  :  Mar 25 - Oct 28
  2013  :  Mar 31 - Oct 27
  2014  :  Mar 30 - Oct 26 - ....
  2015  :  Mar 29 - Oct 25 - 2043, 2071, 2099; 2111, ....
  2016  :  Mar 27 - Oct 30 - 2044, 2072. ----. 2112, ....
  2017  :  Mar 26 - Oct 29 - ....
  2018  :  Mar 25 - Oct 28
  2019  :  Mar 31 - Oct 27
  2020  :  Mar 29 - Oct 25
  2021  :  Mar 28 - Oct 31
  2022  :  Mar 27 - Oct 30
  2023  :  Mar 26 - Oct 29
  2024  :  Mar 31 - Oct 27
  2025  :  Mar 30 - Oct 26
  2026  :  Mar 29 - Oct 25
  2027  :  Mar 28 - Oct 31
  2028  :  Mar 26 - Oct 29
  2029  :  Mar 25 - Oct 28
  2030  :  Mar 31 - Oct 27 - ....
  2031  :  Mar 30 - Oct 26 - 2059, 2087. ----. 2127, ....
  2032  :  Mar 28 - Oct 31 - 2060, 2088; 2100, 2128, ....
  2033  :  Mar 27 - Oct 30 - 2061, 2089; 2101, ....
  2034  :  Mar 26 - Oct 29 - 2062, 2090; 2102, ....
  2035  :  Mar 25 - Oct 28 - 2063, 2091; 2103, ....

External Links

Links to other Summer Time / Daylight Saving (etc.) sites :-

• UK :-
  • HMG :-
    • OPSI : The Summer Time Order 1997 - Statutory Instrument 1997 No. 2982
    • HMSO : The Summer Time Order 2002 - Statutory Instrument 2002 No. 262
    • BERR : BRITISH SUMMER TIME - Dates, links
  • NPL :-
    • Summer Time Dates - for current years (PDF)
    • Archive of Summer time dates - from 1916. Includes Double Summer Time
  • Joseph Myers - "History of legal time in Britain"
  • NMM, UK - Summer Time
  • Greenwich 2000
  • Whitaker's Almanac
  • BST FAQ
  • Barney Wol - Summer Time
• EU :-
  • 31997L0044 Eighth Directive 97/44/EC of the European Parliament and of the Council of 22 July 1997 on summer-time arrangements
  • 32000L0084 Ninth Directive 2000/84/EC of the European Parliament and of the Council of 19 January 2001 on summer-time arrangements
• Europe :-
  • Daten und Sommerzeiten in Deutschland, Österreich und der Schweiz
  • Petite histoire de l'heure d'été
• NA :-
  • Canada, NRC, Time Services FAQ
  • USA, NIST, When does Daylight Saving Time/Standard Time begin?
  • At webexhibits, via : Calendars ?
  • I don't know of an authoritative, detailed, updated Web page for USA; at USNO maybe
  • US & general Summer Time, MiningCo
  • California Energy Commission - good
  • Eric Weisstein - Daylight Saving Time
  • Microsoft : Daylight Saving Time Help and Support Center
  • Apple : About Daylight Saving Time changes in 2007
  • Jeff Regan - 2007 Time Zone Changes Will Impact Many Computers in Canada, the United States and Bermuda
• World :-
  • Sources for Time Zone and Daylight Saving Time Data - "The TZ database" - with many links
  • Worldwide daylight saving (webexhibits)
  • WorldTime Home page ??
  • Steffen Thorsen, "timeanddate" tables, links, details. Links to worldwide change dates
  • Time Zone, in Markus Kuhn's ISO 8601 page
  • ZIC (elsie) : I rarely use this, preferring to maintain independence; but it is a major source. In UNIX compressed format
  • WorldTimeZone, Alexander Krivenyshev :-
    • Abbreviations
    • Places with Summer Time
  • Wikipedia :-
    • Daylight saving time - somewhat US-biased
    • Daylight saving time around the world - somewhat US-biased
• See also in my Time Zone Links

Errors

The following errors have been seen; they may have been corrected.

• UK Press - often get the change times wrong.