(U.K., E.U., and other)
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. Clocks are therefore put ahead for the Summer. It yields a modest energy saving.
The Summer Time rules for most places (including UK, EU; & NA) imply that each transition date is chosen from seven possibilities and is in one-to-one correspondence with the day-of-week of March 1st, that of Christmas Day, and that of New Year's Eve in the same 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 (seasons are approximate). German Law has said that the duplicated hour shall be referred to as 2A & 2B.
Summer Time is equivalent in effect to a temporary change to a different Time Zone (eastward, usually to the next zone). Note that Time Zones correspond to the local Standard (Winter) time.
Double Summer Time (when 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.
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 integer 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 Order and the Directive have effect indefinitely.
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 dates earlier in October), and 31 weeks for March 25-28 (three dates later). It always ends in ISO Week 43 (yyyy-W43-7), except in years with Sunday February 29th (ends yyyy-W44-7), such as 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).
BST is or has been used for British Summer Time, British Standard Time, Bangkok Standard Time, Brazil Standard Time (now BRT), and Bering Summer Time. And, according to Wikipedia BST, Bangladesh Standard Time, Burma Standard Time, Bissau Standard Time, Burkina Faso Standard 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.
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.
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.
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.
|Year||BST On||BDST On||BDST Off||BST Off|
|1916||G2 C2||"||"||G2 C3|
|1941||n/a||G1 C2||G1 C3||"|
|1947||"||G1 C2||G1 C3||"|
|1981||G1 C1||"||"||G1 C2|
G = Greenwich Time, C = Clock Time ; single digits are Hours a.m.
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.
The EU requires a simultaneous EU-wide seasonal clock change of one hour, which would permit omitting a single Autumn change thereby moving the UK to Paris time. What would the Republic of Ireland do? It is clear that an overall advancement of one hour, to Central European Time, would be very unpopular with, and perhaps not accepted by, the Scots. And perhaps the Irish.
August 2010 : Rebecca Harris MP : Daylight Saving Bill 2010-11. Government should analyse costs and benefits of European Time.
February 2011 : The topic is in the UK media, who seem uncertain as to just what the change would be.
October 2011 : Government is consulting regions : UK clocks change trial being considered, Clocks could go forward by an hour all year.
December 2011 : Lighter Later
January 2012 : MPs have debated the Daylight Savings Bill. The UK would adopt Central European Time, for a three-year trial period. The Bill ran out of time in the Commons.
February 2014 : BBC : UK clocks change trial being considered (28 October 2011) - try CET & CEST for three years?
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.
The diagram shows, in blue, the actual offset from GMT of civil time in Britain, from the first use of Summer Time in 1916 up to 2011. The data initially in the Table was taken from NPL data (2011-02-25) and is viewable below.
When the Table is marked for GB, that line is continued in red, up to at most 2099, using the current (2011) EU rules. The purple-coloured line is for Shackleton's men (I do not know just where it should end).
The buttons directly below the plot are for Pan and Zoom, by altering the last number in each Range control. Those accept Y, Y-M, Y-M-D. The diagram is redrawn for form onchange and page onresize events. The px/h control also resizes the fonts.
The GB plot is valid only since 1880 or thereabouts, when GB formally adopted GMT. Since 1916-10-01 02:00 Dublin Time, time in geographical Ireland has matched time in Great Britain, so this plot applies to Ireland only after then. The Data Table is editable and replaceable - non-integer offsets should work but have not been much tested.
The function MinsFromGMT(D) is variable in order to allow simulating the offset values of another time zone by adding a number of minutes and/or reversing the variation. But, for example, D = new Date(+D+864e6) ; return D.getTimezoneOffset() will shift the dates by ten days.
Note for readers outside the GMT time zone : The plot above should look approximately like this (disregarding the moiré effect, and text adjustments), but with the green comb being a thin line if your location does not use Summer Time. RSVP if not.
The following quotations must be read in full, noting the dates received; later parts can supersede earlier ones.
The following information was received from NPL on 1997-06-03, but any errors are mine.
The arrangements for British Summer Time, BST, are established by Parliament through the Summer Time Act 1972. The start and end dates are specified by Summer Time Orders, the most recent of which (The Summer Time Order 1994) covers the years 1995 to 1997. If there is no Summer Time Order in place for a given year, then the Summer Time Act 1972 applies the following rules :-
"... summer time ... is the period beginning at two o'clock, Greenwich mean time, in the morning of the day after the third Saturday in March or, if that day is Easter day, the day after the second Saturday in March, and ending at two o'clock, Greenwich mean time, in the morning of the day after the fourth Saturday in October".
In practice, however, the dates are set by Summer Time Orders.
There have been moves to harmonise the dates for Summer Time changes throughout Europe in order to simplify travel and communications. Harmonisation was established in 1996 and is in place for 1997. The dates for this year are that BST began at one o'clock, Greenwich mean time, in the morning of 30th March 1997 and will end at one o'clock, Greenwich mean time, in the morning of 26th October 1997. The dates for 1998 and beyond have not yet been agreed. The Summer Time Order 1994 implemented the arrangements specified in the seventh EC Directive on summer time arrangements. There is a proposal for an eighth EC Directive on summer time arrangements to cover the years 1998 to 2001 (Official Journal of the European Communities No. C342/5) but not a final directive, as far as I am aware. I expect the UK government to bring a new Summer Time Order in to being when the eighth Directive is finalised.
I hope the information above will be of help.
Regards, Xxxx Yyyyyyy.
The following was received from NPL on 1998-09-20.
The pattern in recent years has been to establish summer time arrangements for just a few years in advance. The latest (Eighth) EC Directive on summer time arrangements gives the dates for 1998 to 2001 (see the Official Journal of the European Communities no. L 206/62, 1 August 1997). We give the actual dates on our web site.
Indirectly from RGO?
In 1996 all clocks in Europe were changed on the same date for the first time.
From 1998 BST will be kept from the last Sunday in March until the last Sunday in October. This has been adopted as a directive from the European Parliament and will be effective from 1998-2001 inclusive.
1998: March 29 - October 25 1999: March 28 - October 31 2000: March 26 - October 29 2001: March 25 - October 28
All changes to be at 1 am GMT.
Information Leaflet No. 6: `Summer Time', from the Royal Observatory Greenwich, said that the change was at 2 a.m. GMT before 1981, and 1 a.m. GMT thereafter. (Does that apply to BDST?).
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.
European Russia : until 2011-03-27, Moscow was in GMT+3; European Russia also used GMT+4 and probably some GMT+5. Now, GMT+4 GMT+5 GMT+6 (with no Summer Time).
See also in Time Zones.
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.
|Use of EU Summer Time Rules,
Last Sundays of March & October
|EU before 2004-05-01|
|EU from 2004-05-01|
|EU from 2007-01-01|
|EU from 2013-07-01|
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.
A page by Myers includes further information.
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.
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. Turkey has sometimes started Summer Time a day late, to avoid disrupting local events.
Iceland uses only GMT/UTC.
For Russia and other fUSSR, see below.
For 1974 and 1975, the USA in general had an early start to "Summer Time".
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's Daylight Time, and see Wikipedia.
Some US software implemented the 1987-2006 Spring change date as April 8th in years when it should have been April 1st.
See also Risks.
There was (May 2001) a proposal (HR 704) to consider Double Summer Time for the Western USA.
Time in Indiana is particularly complicated.
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 (valid primary authoritative URL wanted). Possible subsection (c) Report (1.42MB PDF)? There has been a recent (Nov 2008) Report.
From 2007, DST thus started three or four weeks earlier than before and ended 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 ... .
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.
Following the USA, all of Canada that uses DST has changed to the new dates; I do not know when Canadian Rules formally changed.
Both changing as for the USA. Also maybe Turks and Caicos Islands.
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.
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.
The International Space Station uses Greenwich Time.
Russia changed on the EU dates (the last Sunday of March and the last Sunday of October) before mid-2011. I believe that all parts of the country changed at the same local time. The non-European parts of the former USSR seemed to match Russia's changes.
Russia will not change after 2011-03-27 : Russia to stop setting clocks back in winter: Medvedev. It seems that, formally, their Time Zones have not changed but they now have permanent Summer Time. See Time in Russia.
Russian Space Mission Control kept Moscow Standard Time (GMT+3) all year round. It should have changed to GMT+4 on 2011-03-27. Railways in contiguous Russia use Moscow Time, too.
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.
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 (WA had 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. See in Oddities.
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 Time in Australia and, via NSW Lawlink, Daylight Saving in New South Wales.
Chile, Paraguay, Uruguay, and much of Brazil have Summer Time. The Falklands have Summer Time. Antarctica varies.
In Brazil, Summer Time obeys the usual sort of rules, except that the end of Summer Time is deferred by a week when it would otherwise occur during Carnival (which covers the weekend before Lent).
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.
In Muslim countries, Ramadan might affect clock change dates.
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.
Some countries, including Iceland and Ghana, keep GMT all year round.
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 inevitable 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. Within 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.
Parts of this refer specifically to the US DST change in 2007, or use it as an example.
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 ...??? .
USA, 2007-03-01; Canada, matching.
New Zealand changed in winter 2007 - Risks Digest 28.86 refers.
It was suggested that Turkey might move to permanent GMT+2.5 in 2011.
Time stops: Russia abolishes daylight saving time : Russia dropped Winter Time after Summer 2011; or, rather, it moved its Standard Times ahead by one hour on 2011-03-27 and will no longer has Summer Time. So Moscow now uses GMT+4 year-round.
Venezuela changed time zone from GMT-4 to GMT-4.5 in 2007.
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 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.
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.
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.
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.
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.
In at least some versions of Windows, with at least some filing systems types, apparent file date/times can change when the clock change occurs. That can upset some updating logic.
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".
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.
Most places change their clocks exactly on the hour (or half-hour). But, from a Microsoft update, it appears that Beirut changes at 23:59:59.999, perhaps to avoid confusion over which day it is.
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.
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 :-
* St Helena, Burkina Faso, Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, Morocco, Sao Tome, Senegal, Togo, Western Sahara, according to GreenwichMeanTime.
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.
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.
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.
The National Maritime Museum has opened a small display marking the centenary of William Willett's first proposals for Daylight Saving Time, at the Royal Observatory Greenwich during Summer Time 2007.
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) (p.65, my ISBN 1-55821-783-5)).
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.
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".
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.
This section generally assumes that the present or recent (as at 2007) rules hold; be cautious when dealing with dates before the recent past.
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.
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.
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 :-
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 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.
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.
All of Great Britain (probably all of the British Isles, and the Channel Islands too?) keeps London time, with, I haveread, a small exception, variously described : ROT-13 :-
Lord Howe Island (Australia) has a half-hour time shift (and a time zone to itself). See zone.tab and local sources; search the Web for 'Lord_Howe' (sic).
The dates themselves, originally obtained from various sources, are all in agreement with the NPL archive linked below.
British Summer Time is GMT+1; UK civil time is otherwise GMT.
See NPL site.
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 BrStST - Oct 31
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
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 *
Anomalous end date.
1995 UK Mar 26 - Oct 22 * (Ireland matched the UK)
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 2007 : Mar 25 - Oct 28 2008 : Mar 30 - Oct 26 2009 : Mar 29 - Oct 25 2010 : Mar 28 - Oct 31 2011 : Mar 27 - Oct 30
Naturally, with constant Rules disregarding Easter, the date pattern repeats every 28 years from 1996 to 2099, and (shifted) from 2100 to 2199, etc.
2012 : Mar 25 - Oct 28 2040, 2068, 2086 2013 : Mar 31 - Oct 27 2014 : Mar 30 - Oct 26 2015 : Mar 29 - Oct 25 2016 : Mar 27 - Oct 30 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 2032 : Mar 28 - Oct 31 2033 : Mar 27 - Oct 30 2034 : Mar 26 - Oct 29 2035 : Mar 25 - Oct 28 2036 : Mar 30 - Oct 26 2037 : Mar 29 - Oct 25 2038 : Mar 28 - Oct 31 2039 : Mar 27 - Oct 30 2067, 2085
Links to other Summer Time / Daylight Saving (etc.) sites :-
The following errors have been seen; they may have been corrected.