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Daylight saving time (DST), also known as summer time in British English, is the convention of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour in late winter or early spring and are adjusted backward in autumn. DST is used mostly in temperate and polar regions, where summer mornings are longer and have more hours to spare.

Governments often promote DST as an energy conservation measure on the grounds that it substitutes natural summer sunlight for electrical lighting. In some cases, though, DST can increase energy costs (see Criticism below).

Origin

Saving daylight was first mentioned in 1784 by Benjamin Franklin in a satirical essay urging Parisians to get up earlier in order to use sunlight, thus saving wax by not burning candles at night, in the spirit of his proverb "Early to bed and early to rise / Makes a man healthy, wealthy, and wise." Franklin did not mention Daylight Saving Time—he did not propose that clock time be changed.

DST was first seriously proposed in 1907 by William Willett, but he was unable to get the British government to adopt it, despite considerable lobbying.

DST was first put into practice by a national government in Germany 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 US Congress formally established several time zones, which had been in use by railroads and most cities since 1883; at the same time it established DST from the last Sunday in March to the last Sunday in October. The law, however, proved so unpopular, mostly because it obliged people to rise and go to bed earlier than had become customary since the advent of electricity, that it was repealed after 1919, when Congress overrode President Woodrow Wilson's veto of the repeal.

Observation of DST

In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 01:59:59.999… to 03:00 and that day is 23 hours long, whereas in autumn the clock jumps backward from 01:59:59.999… to 01:00, repeating that hour, and the day is 25 hours long. A one hour clock shift is customary, but Lord Howe Island uses a half-hour shift, and twenty-minute and two-hour shifts have occurred in the past.

Clock shifts typically occur near a weekend midnight to lessen disruption to weekday schedules. In some jurisdictions, such as California, bars are required to close at 02:00, which is when the clock shift occurs, leaving bars open for an extra hour once a year—one of the few direct commercial effects. Hospitals and other 24/7 operations change their clocks but remain open.

Start and end dates and times vary with location and year. Starting in 2007, most of the United States and Canada observe DST from the second Sunday in March to the first Sunday in November, with transitions typically at 02:00 local time. The 2007 US change was part of the Energy Policy Act of 2005; previously, from 1987 through 2006, the start and end dates were the first Sunday in April and the last Sunday in October, and Congress retains the right to go back to the previous dates once an energy consumption study is done. Since 1996 the European Union has observed DST from the last Sunday in March to the last Sunday in October, with transitions at 01:00 UTC.

Beginning and ending dates are switched in the southern hemisphere. For example, mainland Chile observes DST from the second Saturday in October to the second Saturday in March, with transitions at 24:00 local time. The time difference between the United Kingdom and mainland Chile may therefore be three, four, or five hours, depending on the date.

Argentina, Iceland, Saskatchewan and other areas skew time zones westward, in effect observing DST year round without complications from DST shifts. The United Kingdom and Ireland experimented with year-round DST from 1968 to 1971 but abandoned it due to its unpopularity, particularly in the north. Spain, the Yukon and other areas both skew time zones and shift clocks, in effect observing double DST in summer.

DST is generally not observed in the tropics, where day lengths do not vary enough to justify it.

Rationales for DST

Willett's 1907 proposal argued that DST increases opportunities for outdoor leisure activities during afternoon sunlight hours. As a builder, he also wanted workers to arrive at construction sites earlier in summer mornings. Similar motivations remain to this day.

Energy conservation has often been used to justify DST. The idea is that artificially delaying sunrise and sunset tends to increase energy consumption in the morning and reduce it in the evening. In theory, the evening reduction outweighs the morning increase. United States Department of Transportation (DOT) studies concluded in 1975 that DST would probably reduce the country's electricity usage by 1% during March and April. A study after Mexico reintroduced DST in 1996 estimated national savings of 0.7% of national electric consumption and reduction of peak load by 500 MW.

An energy argument for observing DST in summer rather than winter is that most people wake up after the summer sun rises regardless of whether DST is in effect, so DST during summer has less need for morning electrical lighting.

The above-mentioned 1975 DOT studies also identified a 0.7% reduction in traffic fatalities and a reduction of 10 to 13% in the violent crime rate of Washington, DC. Other studies have also found a reduction in traffic fatalities, which far outweigh the increases due to clock shifts noted below.

Criticism of DST

DST has two major problem areas: disagreement with the sun and clock shifts.

Complications from disagreement with the sun

Farmers and others whose hours are set by the sun are adversely affected by DST. For example, farmers may miss non-work evening activities due to scheduling conflicts. Many people alter their nominal work schedules (in effect ignoring DST) to coordinate with daylight, TV broadcasts, or remote colleagues.

DST is commonly not observed during winter, because it suffers more from darker mornings: workers may have no sunlit leisure time, and children may need to leave for school in the dark.

Critics argue that the energy savings of DST are overstated, and that DST can sometimes increase energy consumption and peak demand. A 2007 study by the University of California Energy Institute concluded that the Australian experiment with DST during the 2000 Olympics did not decrease overall electricity consumption, but did change usage patterns leading to a higher morning peak load.

DST makes it harder to read a sundial, since one must compensate for DST in addition to time zone and natural discrepancies.

Critics also suggest that DST is, at its heart, government paternalism.

Complications from DST shifts

DST's clock changes have the obvious disadvantage of complexity. People must remember to change their clocks, and to update software when DST rules change. Some computer systems may require downtime or restarting. People who work across time zone boundaries need to keep track of multiple DST rules. Not all locations observe DST. The length of the day becomes variable. Disruption to meetings, travel, or broadcasts is not uncommon when clocks or rules change.

Changing the clocks disrupts sleep patterns. This correlates with decreased economic efficiency. Researchers estimated in 2000 that the daylight saving effect implies a one day loss of $31 billion on the NYSE, AMEX, and NASDAQ alone. Sleep disruption also correlates with a significant increase in fatal accidents the Monday after a Sunday shift, with the larger increase in autumn.

During an autumn transition from 02:00 to 01:00, the clock will read 01:30 twice, leading to confusion in birth certificates and the like.

These problems can be avoided by adopting Coordinated Universal Time (UTC), which is unaffected by DST. However, UTC is unsuitable for many applications due to its unfamiliarity.

Mnemonic

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.

Associated practices

Fire safety officials encourage citizens to use the two annual clock changes as a reminder to check the batteries in smoke and carbon monoxide detectors. This is especially important in autumn, just before the heating and candle season causes an increase in home fires. Similar semiannual tasks include inspecting vehicle lights, updating family disaster plans, checking storage areas for hazardous materials, and reprogramming thermostats.

Computing

Many computer operating systems can change the local time automatically when daylight saving starts and finishes. See the Time zone article for general information on time zones and computer systems.

Unix-like systems

Most Unix-like systems, including GNU/Linux and Mac OS X, use the zoneinfo database, which maps a name like "America/New_York" to a set of time zone and DST transitions appropriate for the named location. Standard library routines access this database so that applications can calculate past and predicted future time stamps even if the time zone offset or DST rules often change. When temporal authorities change the rules, the main zoneinfo database is updated, and these updates can be installed as part of ordinary system maintenance. A process's TZ environment variable specifies the location name.

A few Unix-like systems, notably AIX, support only the TZ values required by POSIX, which specify at most one start and stop rule explicitly in the value. TZ must be changed whenever DST rules change, and the new TZ value applies to all years, mishandling some older time stamps.

Java platform

Java releases since version 1.4 have integrated a zoneinfo-derived database into the JRE. It is separate from the underlying operating system's database, so the JRE must also be updated when DST rules change. It is located in the lib/zi directory. The database of pre-1.4 Java releases supports only a single start and stop rule per setting.

Microsoft Windows systems

Windows Vista supports at most two DST start and stop rules per time zone setting. For example, in Brazil where DST rules commonly change every year, a Vista setting would support the current and previous year's time stamps, while mishandling some older time stamps. Older Microsoft Windows systems usually store only a single start and end rule for each zone; in the same Brazil example, a setting would reliably support only this year's time stamps.

These limitations have caused several problems. For example:

• When the Australian government changed the 2006 DST end date to aid the 2006 Commonwealth Games, Microsoft merely added new time zones with the words "(Commonwealth Games)". This caused problems with many applications, including Microsoft Outlook. One workaround was to modify the start and end dates of each affected time zone with the Microsoft timezone utility, and then either reboot or go to Control Panel > Date and Time > Time Zone and click "OK" to force Windows to refresh its DST internals.
• Before 2005, DST in Israel varied each year and was skipped some years. Windows 95 used DST rules correct for 1995 only, causing problems in later years. In Windows 98 Microsoft gave up and marked Israel as not having DST, causing Israeli users to change their computer clocks manually twice a year. Beginning in 2005 Israel observes DST from the first Friday before April 2 to the Sunday between Rosh Hashanah and Yom Kippur, but Windows zone files cannot represent these dates in a year-independent way. Workarounds include manually switching zone files every year and an unofficial tool that switches zones automatically.

DST 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 Server 2003 the information comes from the time zone database stored in the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\. When the rule changes an update needs to be applied.

Name

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.

Time zone names typically change when DST is observed. In American English "standard" becomes "daylight": for example, Pacific Standard Time (PST) becomes Pacific Daylight Time (PDT). British English uses "summer": for example, Central European Time (CET) becomes Central European Summer Time (CEST). Abbreviations do not always change: for example, many (though not all) Australians say that Eastern Standard Time (EST) becomes Eastern Summer Time (also EST).

Usage by location

Cultural references

• DST inspired the musical standard "(There Ought To Be A) Moonlight Saving Time" by Irving Kahal and Harry Richman. The song was made famous by Guy Lombardo, Maurice Chevalier, and others.
• Author and comedian Anthony Frosh wrote a tongue-in-cheek short story that deals with the apparent hazards of daylight saving as believed by the residents of Perth, Western Australia, titled The Day After Yesterday: The aftermath of daylight saving.
• Homer Simpson arrives to a closed nuclear plant and believes he is "For once...early, except for all those daylight savings times..."

See also

• Daylight saving time around the world
• Time zone
• Zoneinfo

References

1. Benjamin Franklin (1784). "Daylight saving". Journal of Paris. Retrieved 2007-02-16.
2. Seymour Stanton Block (2006-02). "Benjamin Franklin: America's inventor". American History. Retrieved 2007-02-16. {{cite journal}}: Check date values in: |date= (help)
3. William Willett (1907). "The waste of daylight".
4. Philip Johnston (2007-01-22). "Is it time to lighten our darkness?". Retrieved 2007-02-09.
5. Linda L. Lawson (May 24, 2001). "Testimony before the House Science Committee, Energy Subcommittee, concerning daylight saving time and energy conservation". Retrieved 2007-01-31. {{cite journal}}: Cite journal requires |journal= (help)
6. FIDE 1997. Informe de Labores 1997, Mexico City.
7. Susan A. Ferguson (1995-01). "Daylight saving time and motor vehicle crashes: the reduction in pedestrian and vehicle occupant fatalities". American Journal of Public Health. 85 (1). American Public Health Association: 92–95. PMID 7832269. {{cite journal}}: |access-date= requires |url= (help); Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
8. Douglas Coate (2003-04-10). "The effects of daylight and daylight saving time on US pedestrian fatalities and motor vehicle occupant fatalities". Accident Analysis & Prevention. 36 (3). Elsevier: 351–357. doi:10.1016/S0001-4575(03)00015-0. Retrieved 2007-02-14. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
9. Daniel S. Hamermesh; Caitlin Knowles Myers; Mark L. Pocock (2006-07). "Time zones as cues for coordination: latitude, longitude, and Letterman". National Bureau of Economic Research. Retrieved 2007-02-17. {{cite journal}}: Check date values in: |date= (help); Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
10. Heidi G. Yacker (1998-02-09). "Daylight saving time". {{cite journal}}: Cite journal requires |journal= (help)
11. Ryan Kellogg; Hendrik Wolff (2007-01). "Does extending saylight saving time save energy? Evidence from an Australian experiment" (PDF). University of California Energy Institute. Retrieved 2007-01-31. {{cite journal}}: Check date values in: |date= (help); Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
12. Stephen Tong (2007). "Are you prepared for daylight saving time in 2007?". IT Professional. 9 (1). IEEE Computer Society: 36–41. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
13. Mark J. Kamstra (2000-09). "Losing sleep at the market: the daylight saving anomaly". The American Economic Review. 90 (4). American Economic Association: 1005–1011. Retrieved 2007-02-06. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
14. Jason Varughese (2001-01-04). "Fatal accidents following changes in daylight savings time: the American experience". Sleep Medicine. 2 (1). Elsevier: 31–36. doi:10.1016/S1389-9457(00)00032-0. Retrieved 2007-02-06. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
15. "Daylight saving time: rationale and original idea". Retrieved 2007-02-06. {{cite web}}: Unknown parameter |note= ignored (help)
16. "Turn back your clocks". Consumer Reports. 2006-10. {{cite news}}: Check date values in: |date= (help)
17. The Open Group (2004). "Other environment variables". IEEE Std 1003.1. Retrieved 2007-02-16. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
18. "Australian daylight savings changes for Microsoft products for the year 2006". 2006-08-03. Retrieved 2007-02-16.
19. "Microsoft products do not reflect Australian daylight saving time changes for the year 2006". 2006-10-30.
20. "Aussie Microsoft calendar mess". Vol. 11, no. 5. Office Watch. 2006-01-25.
21. "Daylight saving time help and support center". 2007-02-16. Retrieved 2007-02-18.
22. daylight saving time. Merriam-Webster Online Dictionary, accessed July 13, 2006. "called also daylight saving, daylight savings, daylight savings time, daylight time"
23. daylight-saving time. The American Heritage Dictionary of the English Language, 4th ed., accessed July 13, 2006. "Variant Forms: or daylight-savings time"

Further reading

• David Prerau (2005). Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time. Thunder’s Mouth Press. ISBN 1-56025-655-9. The British version, focusing on the UK, is Saving the Daylight: Why We Put the Clocks Forward. Granta Books. ISBN 1-86207-796-7.
• Michael Downing (2005). Spring Forward: The Annual Madness of Daylight Saving Time. Shoemaker & Hoard. ISBN 1-59376-053-1.

External links

• Daylight Saving Time - general history and anecdotes
• Saving Time, Saving Energy - US viewpoint
• Summer Time - UK viewpoint
• Sources for DST data, including histories and predictions
• Extended DST links, advisories and changes for 2007
• 2007 time zone changes will impact many computers in Canada, the United States and Bermuda

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• Time zones