European Summer Time is the variation of standard clock time that is applied in most European countries, not including Iceland, Belarus, Turkey and Russia — in the period between spring and autumn, during which clocks are advanced by one hour from the time observed in the rest of the year, in order to make the most efficient use of seasonal daylight. It corresponds to the notion and practice of "daylight saving time" to be found in many other parts of the world.
European Summer Time is observed across three time zones, beginning at 01:00 UTC/WET (02:00 CET, 03:00 EET) on the last Sunday in March and ending at 01:00 UTC (02:00 WEST, 03:00 CEST, 04:00 EEST) on the last Sunday in October each year. This means that Summer Time lasts, depending upon the calendar year, for either 30 or 31 weeks of the annual 52.
Summer Time was first introduced during the First World War. However, most countries discontinued the practice after the war. It was then restarted in various countries during the Second World War and its immediate aftermath. Again it was widely cancelled by the 1950s, although re-introduced in isolated cases until the late 1960s when the energy crisis began to prompt policy makers to re-introduce the policy across the continent. It has remained in place in most European countries since that time.
Historically the countries of Europe had different practices for observing Summer Time, but this hindered coordination of transport, communications and movements. Starting in 1981 the European Community began issuing directives requiring member states to legislate particular start and end dates for Summer Time.
Since 1981 each directive has specified a transition time of 01:00 UTC and a start date of the last Sunday in March, but the end dates have differed. Successive Directives laid down two dates for the end: one on the last Sunday in September applied by the continental Member States, and the other on the fourth Sunday in October for the United Kingdom and Ireland. In 1996 the end date was changed to the fourth Sunday in October for all countries. In 1998 the end date was adjusted to be the last Sunday in October; this happened to be the same as the previous rule for 1996 and 1997. The ninth directive, currently in force, has made this permanent.
European Summer Time begins (clocks go forward) at 01:00 UTC on the last Sunday in March:
The formula used to calculate the beginning of European Summer Time is
Sunday (31 − ((((5 × y) ÷ 4) + 4) mod 7)) March at 01:00 UTC
European Summer Time ends (clocks go back) at 01:00 UTC on the last Sunday in October:
To calculate the end of European Summer Time, a variant of the formula above is used for October:
Sunday (31 − ((((5 × y) ÷ 4) + 1) mod 7)) October at 01:00 UTC
"Double Summer Time" (two hours ahead of local winter time) has been observed on some occasions, notably in 1921, 1941-5 and 1947. See:
There are three countries that do not use summer time, and keep the same time all year.
Some may be thought of as using "permanent" summer time since they are using time zones allocated to regions further east than themselves. Belarus explicitly decided to stay permanently on (what it formerly called) summer time after 2011.
Spain, France, Belgium, and the Netherlands may also be thought of as observing "summer time" throughout the winter, and "double-summer time" during summer, because of their position far to the west of the central European time zone.
In most of Europe the word Summer is added to the name of each European time zone during this period: thus, in the UTC+01:00 time zone, Central European Time becomes Central European Summer Time (UTC+02:00).
Austria-Hungary used summer time during World War I in 1916, 1917 and 1918, similarly as the German Empire.
Summer time was introduced in Bulgaria in 1979 by a regulation of the Bulgarian Council of Ministers. Bulgaria observes the European Union rules for summer time.
In the Kingdom of Bohemia, summer time was used for three seasons since 1916 to 1918, while being part of the Austro-Hungarian Empire during WWI. During the WWII period, when Protectorate of Bohemia and Moravia became a de facto part of Nazi Germany, summer time was used since 1940. In 1940/1941 and 1941/1942, the summer time was kept continuously even during the winter. Czechoslovakia used summer time since 1945 to 1949 and since 1979 up to 1992. In winter 1946/1947 (since 1 December to 23 February), also winter time (CET−01:00) was used.
So Czechoslovakia used summer time yearly since 1979 and both Czech Republic and Slovakia continued in doing so after the dissolution in 1993.
Czech lands and Slovakia used summer time in 1916, 1917 and 1918 under the Austro-Hungarian Empire during WWI.
Protectorate of Bohemia and Moravia used summer time since 1940. In 1940/1941 and 1941/1942, the summer time was kept continuously even during the winter. The Slovakian State during WWII also used summer time.
Czechoslovakia continued to use the summer time since 1945 to 1949. In winter 1946/1947 (from 1 December to 23 February), also winter time (CET−01:00) was used.
Czechoslovakia used summer time since 1979 up to 1992 (year of its dissolution). Czech Republic and Slovak Republic continue to use the summer time.
Although summer time has been observed in Denmark for the past few decades and its observance will continue in accordance with EU orders, a national association against summer time (Landsforeningen mod Sommertid) still exists.
In Estonia the use of summer time has been strongly criticised and as a result it was not used in 1989–1996 and 2000–2001. It was used under Soviet rule in 1981–1988.
From 1923 until the Second World War France observed summer time from the last Saturday in March until the first Saturday in October. During the Second World War France also observed summer time. However, after the war the practice was abandoned (since the country changed time zones instituting de facto permanent summer time). In 1975, summer time was reimplemented because of the oil crisis.
Since GMT (now UTC) is France's "natural" time zone its use of UTC+1 in winter can be seen as a form of daylight saving time in winter, while its use of Central European Summer Time (UTC+2) in summer can be seen as a form of "double summer time".
France follows EU rules regarding the start and end times and dates for summer time.
Summer time was first introduced during World War I by the German Empire in the years 1916 to 1918. After the end of the war and the proclamation of the Weimar Republic in November 1918, summer time ceased to be observed in peace time. Summer time was reintroduced in 1940, during World War II, in an attempt to save energy for the war economy. After the defeat of Germany, summer time was retained by the occupation powers. In 1945, Berlin and the Soviet Occupation Zone even observed Central European Midsummer Time (Mitteleuropäische Hochsommerzeit, MEHSZ; UTC+03:00); in 1947, all of Germany switched to midsummer time from 11 May to 29 June. After the Federal Republic (West Germany) and the German Democratic Republic (East Germany) were established in 1949, summer time again ceased to be observed in 1950.
In 1978, West Germany decided to re-introduce summer time, following the example set by several neighbouring states in the aftermath of the 1973 oil crisis. However, it only came into effect in 1980, after West and East Germany reached an agreement to simultaneously observe summer time from the last Sunday in March (02:00 CET) to the last Sunday in September (03:00 CEST). Therefore, both German states observed the same time until the German re-unification in 1990, after which the re-unified Germany retained the laws and thus also the Time Act (Zeitgesetz) of West Germany.
Germany follows EU rules regarding the start and end times and dates for summer time.
Summer time was introduced in Hungary first in 1916 and it was observed until 1919. After that summer time was in use between 1941–1949 and 1954-1957. Summer time has been in use again since 1980 and follows EU rules.
Iceland uses UTC but has not used summer time since April 1968. From 1908 to 1968 Iceland used UTC−1. Summer time was used in 1917-1919, 1921 and 1939-1967.
In Ireland, Summer Time, known as Irish Standard Time (IST) is observed during Summer (March to October). IST is sometimes mistaken for "Irish Summer Time", though this is incorrect.
Summer time in Italy was adopted and abolished several times, being observed from 1916 to 1920 and between 1944 and 1948. A law was approved in 1965 that took effect the following year, and made the application of summer time mandatory in the whole country. Since 1996, it has been coordinated with the European Union.
In Norway, summer time was observed in 1916, 1940–45, and 1959-65. The arrangement was controversial, and in 1965 the Norwegian parliament (Stortinget) voted to discontinue the practice. Their neighbour, Sweden, did not use it.
However, in 1980 summer time was reintroduced (together with Sweden and Denmark), and since at least 2002 Norway has followed the European Union in this matter.
In Poland, "the summer time" was observed in the following years:
In the years 1979 - 1995 the last day of summer time was the last Saturday of September. In 1996 it was changed to the last Saturday of October, in order to synchronise with other countries of the EU.
In Portugal, summer time (locally known by "Hora de Verão") was introduced in 1916. In the years 1922, 1923, 1925, 1930, 1933 and from 1967 to 1975 summer time was not applied. For many years the official hour in the Madeira Islands was one hour earlier than that in Lisbon, and the Azores Islands was two hours behind. Today, in the Madeira Islands the official time is the same as that of Lisbon, and in the Azores Islands is one hour behind Lisbon. The start and end dates for summer time in Portugal follow the pattern in the rest of the EU: the last Sunday in March to the last Sunday in October.
Summer time in Romania was originally introduced in 1932 (between 22 May and 2 October). Between 1933 and 1940 summer time started on the first Sunday in April and ended on the first Sunday in October. Summer time was abandoned in 1941 and reintroduced in 1979. Until 1996, with few exceptions, summer time started at the end of March and ended at the end of September. Since 1997, it has started in the last Sunday in March and ended on the last Sunday in October, in accordance with European Union rules.
In Russia, a decree of the Russian Provisional Government introduced summer time (Russian: летнее время) on 30 June 1917 and clocks moved one hour forward. A decree of the Soviet government led to the abandonment of this system five months later: clocks moved one hour back again on 28 December.
From 1930, Decree time had the effect of imposing year-round time-zone advances in the Soviet Union.
Summer time was reintroduced in the USSR (Moscow Summer Time) on 1 April 1981, by a decision of the Council of Ministers of the USSR, and its practice continued into post-Soviet times until recently. The changeover dates in Russia were the same as for other European countries, but clocks were moved forward or back at 02:00 local time in all zones. Thus in Moscow (local time = UTC+3 in winter, UTC+4 in summer), summer time commenced at 02:00 UTC on the day before the last Sunday in March, and ended at 03:00 UTC on the day before the last Sunday in October. (Note that "day before last Sunday" is not the same as "last Saturday" in a month where the last day is a Saturday.)
On 8 February 2011, Russian President Dmitry Medvedev announced cancellation of biannual daylight switches in Russia in favor of a permanent switch to summer time. An hour was added in March 2011 for the last time, and clocks did not move back again. At the same time some of Russia's time zones were consolidated. After this reform many Russian cities have a "standard time" two hours more than would be suggested by their "astronomical time" (because the original standard time was already ahead of astronomical time in many areas).
During his 2012 election campaign, Vladimir Putin proposed re-introducing summer time, as there had been complaints about some workers not seeing any daylight during the winter, since the sun had not risen when going to work. According to a report in the International Herald Tribune, the winter of 2011-12 was remembered as the "darkest winter on record" as a result of the time change. However, Putin later said it would be up to then Prime Minister Medvedev's cabinet to decide how to proceed with a seasonal time shift, and it decided to stay with the 2011 policy.
In Slovakia, formerly part of Czechoslovakia, the authorities introduced summer time (locally known as "Letný čas"), with occasional breaks, in the early 1940s. Annual summer time, however, dates only from 1979. After several years, Slovakia established the rule that summer time begins in the last weekend of March (during the night from Saturday to Sunday) and ends in the last weekend of September. Since 1996, summer time has been prolonged about one month so it lasts until the last weekend in October, in accordance with European Union rules.
Summer time in Slovenia (locally known as "Poletni čas") was introduced on 16 November 1982 when it was one of the Yugoslavia republics. Same law was valid until 1996 when the end of summer time was changed from first Sunday in October to last Sunday in October. In 2006, the European Union standard was adopted and is still used today.
In Sweden summer time was originally introduced on 15 May 1916. Then it proved unpopular, and on 30 September in the same year, Sweden returned to year-round standard time. This situation continued for more than half a century.
On 6 April 1980, Sweden again introduced summer time, and since then summer time has been observed every summer in Sweden. Except for the introduction year 1980, summer time has always started on the last Sunday in March. It ended on the last Sunday in September during the years 1980-1995, and has ended on the last Sunday in October since 1996, following a unification of start/end dates of summer time within the EU as well as in several European countries then outside the EU.
The transit authority Västtrafik changes their clocks at 4 AM, so that the last tram that leaves around 3:30 actually goes 2:30 on last weekend of October for those who have changed their clocks at the legal time. The public transport company SL changes their clocks at the legal time, and runs extra departures during the October transition hour.
The last country in Europe to adopt summer time, in 1981, was Switzerland, despite the fact the summer time was rejected in a federal voting in 1977 by 52.1% of voters. Since 1996 Swiss summer time follows EU regulations. It had been in use in 1941 and 1942.
Turkey observes the EU rules for both the date and the time of its clock changes.
Summer time was introduced in Turkey in 1947, but suspended from 1965 through 1972. Since 1974, Turkey follows European Summer Time.
In 2008, the Turkish Ministry of Energy proposed that Turkey should abolish summer time while at the same time switching to GMT +2.5, originally from 2009 onwards, but when this appeared infeasible, to start in 2011. The plan has not been heard of since.
For the year 2011, Turkey switched to European Summer Time at 3:00 am (03:00) on Monday 28 March, one day later than the rest of Europe, to avoid disrupting the national university-entrance examinations held on 27 March.
Once again, for the year 2014, Turkey switched to European Summer Time at 3:00 am (03:00) on Monday 31 March, one day later than the rest of Europe, to avoid disrupting the local elections held on 30 March.
In 2015, Turkey delayed the switch from European Summer Time by 2 weeks, to 4:00 am (04:00) on Sunday 8 November, two weeks later than the rest of Europe, due to the calling of a snap general election on Sunday, 1 November.
In 2016, Turkey scrapped winter time, by switching to New Turkey Time. This means permanent UTC+3, which was used during summer time in Turkey. The switch was on 0:00 am (00:00) on Thursday 8 September, in reality stopping switches between summer and winter time. 
On 20 September 2011 the Verkhovna Rada (Ukrainian parliament) voted not to return from Eastern European Summer Time to Eastern European Time. This change would have had the effect of moving Ukraine into the Further-eastern European Time zone UTC+03:00 along with Belarus and western Russia (which do not observe summer time). However, on 18 October 2011 the Parliament canceled these plans and the country returned to Eastern European Time as scheduled. 295 MPs voted in favour out of 349 registered MPs.
In the United Kingdom local time during this period is known as British Summer Time (BST) (UTC+01:00) while local time during the rest of the year is normally referred to as Greenwich Mean Time (GMT). More generally British Summer Time is part of Western European Summer Time.
Legislation: Summer Time Act 1916; Summer Time Act 1922; Time (Ireland) Act, 1916; Summer Time Act, 1925; Emergency Powers (Defence) Act, 1939; The Summer Time Order 1964; The Summer Time Order 1967; Summer Time Act 1972; The Summer Time Order 1997; The Summer Time Order 2002.
Since 1996 all clocks in the European Union, of which the UK is a member state, have changed on same dates and at the same time, 01:00 GMT.