The idea of starting the day earlier in the summer to take advantage of the earlier sunrise was first put forth by Benjamin Franklin in his essay An Economical Project, which he wrote in 1784 during a sojourn in Paris as a delegate.

The concept behind the change of time is that when the clocks are set forward during spring, we make fuller use of the sunlight available over the day, thus reducing the need for artificial lighting in the evening. This is offset during the winter months by the need for artificial lighting in the morning, as the sun rises later. I, personally, do not see why we cannot keep to the Summer Time (as it is called in most regions other than the United States) for the entire year, as any energy required in the winter mornings would then be conserved in the winter evenings and the additional advantages to a later sunset would still be valid.

In 1947, it was written:

I don't really care how time is reckoned so long as there is some agreement about it, but I object to being told that I am saving daylight when my reason tells me that I am doing nothing of the kind. I even object to the implication that I am wasting something valuable if I stay in bed after the sun has risen. As an admirer of moonlight I resent the bossy insistence of those who want to reduce my time for enjoying it. At the back of the Daylight Saving scheme I detect the bony, blue-fingered hand of Puritanism, eager to push people into bed earlier, and get them up earlier, to make them healthy, wealthy and wise in spite of themselves.
A London builder, William Willet was the first to seriously consider the concept, and wrote a pamphlet in 1907 called Waste of Daylight. He proposed that during the four summer months, advancing clocks by 20 minutes on Sundays, and doing the reverse during the winter. In his pamphlet, he observed:
Everyone appreciates the long, light evenings. Everyone laments their shortage as Autumn approaches; and everyone has given utterance to regret that the clear, bright light of an early morning during Spring and Summer months is so seldom seen or used.

In Britain, the first bill was drafted in 1909 and met with opposition, particularly from the farming community. Summer Time was adopted in May of 1916, the year after Willit's death. During World War II, all clocks were set forward one hour from Greenwich Mean Time (GMT), meaning that during the summer, they were set two hours forward, referred to as Double Summer Time.

Britain was the first country to set a standard time throughout a region, the last holdout being the legal system, which adopted the standardized GMT by statute in 1880. In both Britain, the US and Canada it was the railroads that first instituted and were instrumental in the eventual adoption of standardized time and time zones. But while standardized time in time zones was instituted by the railroads in 1883, it was not established in US law until The Standard Time Act of 1918. When the Department of Transportation was created in 1966, the responsibility of maintaining time standards and zones was reassigned.

Under The Uniform Time Act of 1966, Daylight Saving Time (DST) was legislated to begin on the last Sunday of April and end on the last Sunday of October. In 1986, the law was amended so that DST would begin on the first Sunday of April. By the passage of state law Arizona and Hawaii do not observe DST. Indiana, which is split between the Eastern and Central time zones, legislated the adoption of DST in April of 2005 by a narrow margin. While there is no law that states that DST must be observed, if a state chooses to observe it, it must follow the dates already set forth.

In 2007, the observation of DST in the US was changed to begin on the second Sunday in March and end on the first Sunday of November.

The province of Saskatchewan in Canada does not observe DST as well as parts of British Columbia.

There are approximately 70 countries around the world that observe DST, Summer Time or a similar time change to conserve energy. The tropics do not observe a time change as their latitude makes for little change in day length over the course of the year. In 1996, the European Union (EU) standardized Summer Time to run from the last Sunday of March to the last Sunday of October throughout the EU. Also in 1996, Mexico asopted the same DST rules as the US. Russia, being a higher latitude, is one hour ahead during the winter and then two hours forward for observance of Summer Time. The USSR set their clocks forward one hour in 1930 and resumed the change to Summer Time in 1981. While Russia maintains this arrangement, the other former soviet countries have returned to only the one hour during Summer Time. In 1999, Jordan implemented Summer Time year round. The southern hemisphere observes their Summer Time from October to March. China, which spans 5 time zones, is GMT +8 for the entire country and does not observe DST. In Japan, DST was instituted by occupying forces, but abandoned in 1952 due to extreme opposition by farmers.

A study in the US during the years of 1974 and 1975 confirmed the fact that the observance of DST conserves energy with approximately 10,000 barrels of oil each day it was observed. Also about 1 life, 40 injuries and more than $400,000.00 in accident costs for every other day of observance, as well as reduced crime rates.


Thanks go out to Apatrix for additional information.

Many countries and regions are operating Daylight Saving Time, which means for those of us in the Northern hemisphere, moving the clocks forward 1 hour in March/April and 1 hour back in October/November. Some Australasian regions also have daylight saving, but for these, the DST starts in October (clocks move forward) and finishes in March (clocks move back).

In Europe, the change is on the last Sunday in March, but a week later, i.e. the first Sunday in April for America and Canada. The change in October takes place on the last Sunday in October. From 2007, policy in the USA (with Canada following suit) will be changing to the 2nd Sunday in March and the 1st Sunday in November, under the Energy Policy Act of 2005. This change will be monitored by the US Department of Energy, and Congress retain the right to change it back if no significant energy savings are being made.

The change to and from DST takes place on the Sunday at 02:00 (am). Some atomic clocks such as the one at Boulder, Colorado encode a flag to indicate DST in their time transmissions, which receiving clocks can choose to process or ignore.

Daylight Saving and computers

The simplest appliances with clocks operate solely in local time, and need manually adjusting when DST changes. Modern video recorders have the logic and data of time zones, hence change automatically.

The Windows operating system has handled DST since Windows 95. This version (and 98) has a time dialog window opening with the message "Windows has updated your clock as a result of Daylight Saving Time", but this doesn't occur on NT based operating systems.

There is a potential problem with machines that are running time stamped transaction updates across the October/November change, in that the clock jumps backwards. This can result in transactions ordered by time stamp getting out of sequence. The workaround in most cases is to bring the applications down over the period of DST change.

Unix operating systems work differently. The clock on a Unix box operates in Universal Coordinated Time (UTC), with a time zone offset. When DST changes, there is no change to the box's time, merely to the offset. Databases use UTC rather than local time, hence avoid the transaction problem.

The OpenVMS operating system has a different solution, with the DTSS component. Many legacy databases and transaction processing systems use system time, which is local time. DTSS implements UTC and time zones, running alongside system time. For daylight saving changes, DTSS operates a continuous drift between 00:00 and 04:00 on the day the DST change happens. This causes the system clock to run more quickly or more slowly during this interval, but timed events still happen in sequence. For a Spring change, the clock runs faster than normal, advancing 5 hours for 4 hours of elapsed time. For an Autumn change, the clock is only advanced 3 hours. NTP is based on DTSS.

Sources:

Thank you Wntrmute, rootbeer277 and StrawberryFrog for suggestions that have been included in this writeup.

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