At the equator the day lasts 12 hours with slight annual variation. As latitudes increase so does the variation between winter and summer day-length. For thousands of years this was not a problem as people could organise their days locally around the sun, moon and season. However, industrialisation and urbanisation led to common working hours and by the 19th century many people lived by the clock. Standard times were adopted in response to the spread of railways and telegraphic communications. In 1880 in Britain the clocks were set to Greenwich Mean Time (GMT) i.e. when the sun is at its highest in the sky it is 12 noon at Greenwich.
Some people realised that resetting the clocks could provide social and economic benefit by swapping light from normal sleep time to the evenings. In 1818 the progressive factory owner Robert Owen introduced daylight saving at his New Lanark cotton mills by putting the clocks forward half an hour in the summer. By the start of the 20th Century people like William Willetts were lobbying governments to create a system for incremental clock changes. His scheme was ridiculed and much opposed and although a Daylight Saving Bill was introduced in 1909 there was still not enough support.
Daylight-saving was first enacted in Europe in 1916 as a response to war. To conserve fuel Germany and Austria advanced the clocks by one hour between 30th April and 1st October. The United Kingdom quickly adopted daylight-saving and others followed including Australia, Belgium, Denmark, France, Italy, Luxembourg, Netherlands, Norway, Portugal, Sweden, Turkey and the USA. In Britain the experiment became the norm, and Parliament adopted British Summer Time as a permanent fixture. During the second world war Summer Time was in force all year round. The clocks were put forward an extra hour in the summer to maximise the benefits of natural light. This (GMT+2) was repeated in 1947 when the country was in the grip of post-war austerity.
From 1968 to 1971 Britain experimented with keeping BST all year round. It was dubbed British Standard Time. The motive was to help British industry work more closely with Europe. The experiment failed because the United Kingdom reaches high latitudes and people in the northern parts were deprived of light for most of the morning in the short winter days. It was decided to revert to keeping GMT in winter and BST in summer from 1972.
This Act to consolidate all previous legislation defined British Summer Time as:
During the 20th century many diplomatic efforts were made to harmonise time settings in Europe. Finally the EU (then the European Economic Community) began to legislate for unified Daylight Saving periods.
Since 1981 EU (formerly EEC and EC) Directives have prescribed the dates of summer time in all member states. In 1996 all clocks in Europe were changed on the same date for the first time. The current arrangement was set out in Directive 2000/84/EC of the European Parliament and of the Council of 19 January 2001 on summer-time arrangements.
The Directive was implemented in English Law by Order 2002/262 The Summer Time Order 2002 One new effect of the Order is that clocks will change on Easter Day if it falls on the last Sunday in March. Previously Easter had been avoided.
There is an interesting article on the Royal Museums Greenwich website.