Daylight saving time
Learn more about Daylight saving time
- For a list of various nations' usage of DST, see Daylight saving time around the world.
Daylight saving time (DST), also known as summer time, is a conventional local time adopted by many countries of the world on a seasonal basis. Most commonly DST is obtained by adjusting the official local time forward, by one hour, for the spring, summer, and early autumn periods.
DST is mostly used in temperate regions, due to the considerable variation in the amount of daylight versus darkness across the seasons in those regions.
Governments often promote it as an energy conservation measure, on the grounds that it allows more effective use of natural sunlight resource in summer time. Since there is less darkness in the "waking day", there is less use of electric lights. Some opponents reject this argument (see below, Criticism).
Europeans commonly refer to the system as "summer time": Irish Summer Time, British Summer Time, and European Summer Time. This is reflected in the time zones' names as well, e.g., Central European Time (CET) becomes Central European Summer Time (CEST).
The word "summer" in this context includes most of spring after the spring equinox and a large part of autumn. Likewise, the word "winter" here includes part of autumn and a few weeks in spring. This varies by time zone, of course, and can change over time as well.
DST was first mentioned in 1784 by Benjamin Franklin in a letter to the editors of the Journal of Paris.<ref>Read the full text of Essay on Daylight Saving.</ref> However, as the satirical article was humorous, it is extremely clear Franklin did not seriously propose that the French adopt it. The mere suggestion that a tax be levied on those who have their shades drawn during daylight hours, or simply that people should get up and go to bed earlier is ludicrous.<ref>Franklin's dictum "early to bed, early to rise, makes a man healthy, wealthy, and wise" urged his fellow countrymen to work during daylight and sleep after dark, saving money on candles. Benjamin Franklin: America's Inventor </ref>
The idea of DST was first put into practice by the German government during the First World War, between April 30 1916 and October 1 1916. Shortly afterward, the United Kingdom followed suit, first adopting it between May 21 and October 1 1916. On June 17 1917 Newfoundland became the first North American jurisdiction to adopt DST with the passing of the Daylight Saving Act of 1917. On March 19 1918, the U.S. Congress formally established several time zones, which had been in use by railroads and most cities since 1883; at the same time they made DST official, effective March 31, for the remainder of World War I. It was observed for seven months in 1918 and 1919. The law, however, proved so unpopular, mostly because people rose and went to bed earlier than in current times, that it was repealed in 1919, when Congress overrode President Woodrow Wilson's veto of the repeal.
 Observation of DST
DST is generally a temperate zone practice; day lengths in the tropics do not vary enough to justify DST. The amount of the time shift varies, but one hour is the most common. The dates of the beginning and ending of DST also vary by country. With a few exceptions, switchovers between standard time and DST generally occur in the early hours of a Sunday morning, because doing so then causes less disruption than a change on a weekday would.
DST commonly begins in the northern hemisphere on the last Sunday in March or the first Sunday in April, and ends on the last Sunday in October. However, due to the Energy Policy Act of 2005, beginning in 2007, the United States will begin observing DST from the second Sunday in March until the first Sunday in November. (Studies will determine if this remains permanent.) Most of Canada will also observe the new period to avoid possible economic losses from confusion with the United States. Since 2002, the European Union has fixed the last Sunday in March and the last Sunday in October as start and end dates (European Summer Time).
In the southern hemisphere, the beginning and ending dates are switched; therefore, the time difference between the United Kingdom and Chile may be three, four, or five hours, depending on the time of year.
 Rationales for DST
One of the major reasons given for observing DST is energy conservation. Theoretically, the amount of residential electricity needed in evening hours is dependent both on when the sun sets and when people go to bed. Because people tend to observe the same bedtime year-round, by artificially moving sunset one hour later, the amount of energy used is theoretically reduced. A 1975 United States Department of Transportation study showed that DST would theoretically reduce the country's electricity usage by 1% from March to April, if implemented during these months.<ref>Study by US Department of Transportation</ref> These numbers have been supported in Mexico, which began implementing daylight saving time in 1996. Evaluations show national savings of 0.7% of national electric consumption (1.3 billion KWh (TWh)) and reduction of peak load by 500MW.<ref>FIDE 1997. Informe de Labores 1997,Mexico City.</ref>
Part of the reason that it is normally observed only in the early spring, summer, and early autumn instead of the winter months is that the amount of energy saved by experiencing sunset one hour later would be negated by the increased need for artificial morning lighting due to a later sunrise. During the summer most people would wake up after the sun rises, regardless of whether daylight saving time is in effect or not, so there is no increased need for morning lighting to offset the afternoon drop in energy usage. Another reason for not observing daylight saving time in the winter is concern about children walking to school in the dark.
Another argued benefit of DST is increased opportunities for outdoor activities. Most people plan outdoor activities during sunlight hours. Other benefits cited include prevention of traffic injuries (by allowing more people to return home from work or school in daylight), and crime reduction (by reducing people's risk of being targets of crimes that are more common in dark areas).
When the U.S. went on extended DST in 1974 and 1975 in response to the 1973 energy crisis, Department of Transportation studies found that observing DST in March and April saved 10,000 barrels of oil a day, and prevented about 2,000 traffic injuries and 50 fatalities saving about U.S. $28 million in traffic costs.
 Criticism of DST
DST is not universally accepted and many localities do not observe it. Opponents claim that there is not enough benefit to justify the need to adjust clocks twice every year. The disruption in sleep patterns associated with setting clocks either forward or backward correlates with a small increase in the number of fatal road accidents<ref>"Fatal accidents following changes in daylight savings [sic time: the American experience"], Jason Varughese and Richard P. Allen. March 28, 2000. URL accessed August 15, 2006.</ref> (cf. above estimate of net decrease in fatal road accidents of 50) as well as lost productivity as sleep-disrupted workers adjust to the schedule change.<ref> "Sleep Deprivation, Psychosis and Mental Efficiency"], Stanley Corren, PhD. March 1998. URL accessed September 22, 2006.</ref> It is also noted that much effort is spent reminding everyone twice a year of the change, and thousands are inconvenienced by showing up at the wrong time when they forget. Since DST exchanges morning daylight for evening daylight, late sunrises occur when DST is in effect either too far before the vernal equinox or too far after the autumnal equinox and darkness in the morning can be undesirable for early risers like schoolchildren and workers who must awaken at 6:30 a.m. or earlier.
There is also a question whether the decrease in lighting costs justifies the increase in summertime air conditioning costs. Workers arriving home to an empty house during hotter hours will need to use more energy to cool their house.
It is also speculated that one of the benefits—more afternoon sun—would also actually increase energy consumption as people get into their cars to enjoy more time for shopping and the like.
DST's twice-annual shifts in recorded time cause legal and business-operational complications, as shown in the following examples. During a North American time change, a fall night during which clocks are reset from 2 a.m. DST to 1 a.m. Standard Time, times between 1 a.m. and 2 a.m. will occur twice, causing confusion in transport schedules, payment systems, etc. DST's annual autumn shift in recorded time—which causes an hour of the same numerical name to be recorded twice—also means that people born during one of those two hours have no way to know which of standard time or DST was used to record the time of their birth, unless someone such as a parent makes a note of it; birth certificates rarely keep track of this. A British politician, Lord Balfour, noted the legal complications in British law: "Supposing some unfortunate Lady was confined with twins and the first child was born 10 minutes before 3 o'clock British Summer Time. ... the time of birth of the two children would be reversed. ... Such an alteration might conceivably affect the property and titles in that House."<ref name="Balfour">The Epoch Times "Daylight Saving Time Change Upcoming" Lord Balfour quote. "Supposing some unfortunate Lady was confined with twins..."</ref>
Daylight saving time also causes much confusion with international business, people who commute across time zones, and computer networks that span multiple time zones. One particular problem for scheduling systems is that it makes the length of a day variable. Each year there is one 23 hour day and one 25 hour day, causing display and time tracking problems, especially when coordinating events between time zones.
Some studies do show that changing the clock increases the traffic accident rate.<ref name=autoaccidents>Ferguson, S.A. et al. (1995) Daylight saving time and motor vehicle crashes: the reduction in pedestrian and vehicle occupant fatalities. American Journal of Public Health 85, 92–95.</ref> Following the spring shift to DST, when one hour of sleep is lost, there is a measurable increase in the number of traffic accidents that result in fatalities.
People who work nights often have an extra hassle logging how many hours they worked, since it will be either one hour more or one hour less than the simple difference in start/stop times.
DST is particularly unpopular among people working in agriculture<ref> "History & Info - Daylight Saving Time", 21 October 2006,</ref> because they must rise with the sun regardless of what the clock says, and thus the people are placed out of synchronization with the rest of the community, including school times, broadcast schedules, and the like.
Other critics suggest that DST is, at its heart, government paternalism and that people rise in the morning as a matter of choice because many people enjoy night-time hours and their jobs do not require them to make the most of daylight. Different people start their day at different times (office workers start their day later than factory workers, who start their day later than farm workers), regardless of daylight saving time.
The mnemonic "spring forward, fall back" (also "spring ahead, fall back," "spring ahead, fall behind," or "spring up, fall back") helps people remember how to reset clocks when the time changes. This uses the word "fall" to mean "autumn," a usage that is very common in American and Canadian English, though not so in British English. It is easily remembered by thinking that you more commonly fall backward, than forward.
 Associated practices
Fire safety officials in Australia, Canada, New Zealand, and the United States encourage citizens to use the two annual time changes as a reminder to check the batteries in home and office fire alarms and smoke detectors. For example, the Country Fire Authority of Victoria in Australia has been running a program called "Change Your Clock, Change Your Smoke Alarm Battery" for several years. This is especially important in autumn, just before the heating season causes an increase in home fires.
Most modern computer operating systems include the capability to automatically change the local time when daylight saving starts and finishes. See the Time zone article for general information on time zones and computer systems. Israel, until a few years ago, observed DST on different dates each year, and as its new system relies on the Jewish lunar calendar, most computers do not handle Israel Summer Time (IST).
 Windows systems
The time zone database in most Windows-based computer systems stores only a single start and end rule for each zone, and daylight saving information is stored in the registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\, under the TZI registry value. (In Windows XP and Windows 2003, time-zone information is stored in the registry key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\). For example, DST ends on the last Sunday in October, regardless of year. When the rule changes (e.g. Australian DST ending one week later than usual in 2006, or DST being extended in the United States starting in 2007), an update needs to be applied. In the case of a single-year anomaly, a new time zone is created and used. Before the following year, the time zone will have to be switched back to the original. For permanent rule changes, the rule definition for the time zone can be changed without requiring a new time zone to be set up.
One of the problems of this approach is that software that uses time zone information will get incorrect results if referring to a year with rules that are different from those currently in the database. A good example is the Lotus Notes calendar system, which stores event times in UTC. Events created with calendar dates near DST start and end dates can have their local time interpretation changed after the time zone database is updated (i.e. after an operating system update is applied). Another issue was highlighted when the Australian government changed daylight saving time to end on April 2 instead of March 26, because of the 2006 Commonwealth Games. Microsoft did not modify the start and end rule for the time zones affected, but instead added new timezones with the words "(Commonwealth Games)"<ref name="MS-TZ-CommGames2006">Microsoft Download: Australian Daylight Savings Changes for Microsoft Products for the Year 2006; & MS-KB article 909915, "Microsoft products do not reflect Australian daylight saving time changes for the year 2006" - February 27, 2006</ref> which caused various issues with many software applications, including Microsoft Outlook<ref name="OfficeWatch-v11-n5">Aussie Microsoft calendar mess. Office Watch, volume 11 no. 5, 25 January 2006.</ref> and several accounting packages. Workarounds for the issue were to use the Microsoft utility
timezone to modify the start and finish of each affected timezone, then either reboot the computer or go into Date and Time in the Control Panel, click on the Time Zone tab and click on OK to force Windows to refresh its daylight saving time information.
 Unix systems
Unix systems (including Linux and Mac OS X) typically use the zoneinfo utility which allows a single time zone to have multiple DST rules to handle changes from year to year. As soon as a rule change is announced, it can be safely added to the system. All the standard library routines which calculate times access this database, so software that queries whether a particular date will have DST in effect (for the time zone of the process) will get the correct answers as long as the time zone rule is correct for the year in question.
 Java platform
Java uses a similar database to Unix, so rules for multiple years (not just the current year) can be represented. This database is integrated into the JRE and is separate from the underlying operating system's time zone database, so the JRE must also be updated when DST rules change.
In the normative form of the name, "daylight saving" is a compound adjective that modifies "time." A common variant is daylight savings time, which is frequently heard in speech and appears in some dictionaries.<ref>"daylight saving time." Merriam-Webster Online Dictionary, accessed July 13, 2006. "called also daylight saving, daylight savings, daylight savings time, daylight time"</ref><ref>daylight-saving time. The American Heritage Dictionary of the English Language, 4th ed., accessed July 13, 2006. "Variant Forms: or daylight-savings time"</ref>
 Hour interchange
- When DST begins, the day loses one hour (hour interchange of -1). At this date, a full hour is skipped and does not exist either before or after the transition, so this date includes only 23 hours.
- When DST ends, the day gains one hour (hour interchange of +1). At this date, the same hour occurs twice, first in daylight (summer) time, and then in standard (winter) time, so this date includes 25 hours. As such, October is the longest month in those places where DST ends in October, being 31 days and 1 hour long. In the Southern hemisphere, where DST commonly ends in March, that month is 31 days and 1 hour long.
- Note that some areas apply an offset of only one half-hour for their DST, such as Lord Howe Island: At the date of transition to DST, the legal day counts only 23 hours and 30 minutes, and at the date of transition back to standard time, the legal day counts 24 hours and 30 minutes.
 Usage by location
 See also
- Prerau, David. Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time (Thunder’s Mouth Press; ISBN 1-56025-655-9)
- Prerau, David. Saving the Daylight: Why We Put the Clocks Forward (Granta Books; ISBN 1-86207-796-7) — The Story of Summer Time/Daylight Saving time with a focus on the UK
- Downing, Michael. Spring Forward: The Annual Madness of Daylight Saving Time (Shoemaker & Hoard; ISBN 1-59376-053-1)
 External links
- A Brief History of Daylight Saving Time
- Congressional Research Service (CRS) Reports regarding DST
- DST by WebExhibits
- EU directive 2000/84/EC
- Saving Time, Saving Energy - United States DST schedule (including new rules starting in 2007), explanation, history
- Impacts on Operating Systems and Applications due to new DST schedule starting in 2007
- Sources for daylight saving time data, including histories and predictions
- Summer Time by JR Stockton. With future changeover dates EU (until 2007 final, from then extrapolating)
- Sleep deficit and accidents
- National Association of Standard Time
- PhotoSydney: Daylight Saving examines daylight saving in Australia.
- www.TimeAndDate.com - Time calculation services
- www.thetimeworld.com Daylight-saving time with future changeover dates world-wide
- "Clocks Back, Accidents Up- an argument against Daylight Savings Time by journalist Mike Rutherford on AOL
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