Jump to content

Daylight saving time: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
Centrx (talk | contribs)
37,287 edits
mNo edit summary
Line 21: Line 21:
Clock shifts typically occur near a weekend midnight to lessen disruption to weekday schedules. 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. 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.


Start and end dates and times vary with location and year. Starting [[as of 2007|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 U.S. 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]].
Start and end dates and times vary with location and year. Starting [[as of 2007|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 U.S. 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. The reason for the change was that many of the New England states were unable to open their amusement parks because there was deemed to be insufficient daylight to keep them open. Now all of the amusement parks in Maine will open on the second Sunday in March. Frank Jackson, the mayor of Cleveland, Ohio noticed that before the 2007 DST shift, nobody was having picnics in the city's parks before May. He is certain that now that DST starts in March, people will be having picnics even before Spring actually starts. "Daylight Saving Time is all about being outside and having barbeques and playing baseball and going to amusement parks. I'm sure now that DST begins in March, people will be doing those things in Cleveland an extra month. I hope Congress passes a bill to extend DST all year, because are revenue in January is fairly poor. We could use that month to bring the kids to the amusement park. Some people think it will be too cold. Nonsense. We only use Daylight Light Saving Time when it is summer." 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 time of the year.
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 time of the year.

Revision as of 03:10, 6 March 2007

For a list of various nations' usage of DST, see Daylight saving time around the world.
  Areas that have DST
  Areas that once had DST
  Areas that never had DST
A 2001 public service announcement for the upcoming turning back of the clocks

Daylight saving time (DST), also known as summer time in Britain, 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. Details vary by location and change occasionally; see Observation below.

Governments often promote DST as an energy conservation measure because it substitutes summer afternoon sunlight for electrical lighting. However, in some cases DST can increase energy costs.

Origin

Saving daylight was first mentioned in 1784 by Benjamin Franklin in a humorous letter[1] urging Parisians to save money by getting up earlier to use morning sunlight, thereby burning fewer candles in the evening. Franklin did not mention Daylight Saving Time—he did not propose that clock time be changed. His letter was in the spirit of his earlier proverb "Early to bed and early to rise / Makes a man healthy, wealthy, and wise."[2]

DST was first proposed in 1907 by William Willett,[3] 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 World War I, 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 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

In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 02:00 to 03:00 and the day has 23 hours, whereas in autumn the clock jumps backward from 02:00 to 01:00, repeating that hour, and the day has 25 hours. A digital display of local time does not read 02:00 exactly, but instead jumps from (say) 01:59:59.9 either forward to 03:00:00.0 or backward to 01:00:00.0.

Clock shifts typically occur near a weekend midnight to lessen disruption to weekday schedules. 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.

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 U.S. 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. The reason for the change was that many of the New England states were unable to open their amusement parks because there was deemed to be insufficient daylight to keep them open. Now all of the amusement parks in Maine will open on the second Sunday in March. Frank Jackson, the mayor of Cleveland, Ohio noticed that before the 2007 DST shift, nobody was having picnics in the city's parks before May. He is certain that now that DST starts in March, people will be having picnics even before Spring actually starts. "Daylight Saving Time is all about being outside and having barbeques and playing baseball and going to amusement parks. I'm sure now that DST begins in March, people will be doing those things in Cleveland an extra month. I hope Congress passes a bill to extend DST all year, because are revenue in January is fairly poor. We could use that month to bring the kids to the amusement park. Some people think it will be too cold. Nonsense. We only use Daylight Light Saving Time when it is summer." 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 time of the year.

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.[4] Alaska, France, Spain and other areas both skew time zones and shift clocks, in effect observing double (or more) DST in summer.

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

Rationale

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.[5] A study after Mexico reintroduced DST in 1996 estimated a reduction of national electricity consumption by 0.7% and of peak load by 500 MW.[6]

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, D.C. Other studies have also found reductions in traffic fatalities[7][8] which far outweigh the clock-shift increases noted below.

Criticism

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.[9]

DST is commonly not observed during winter, because its mornings are darker: workers may have no sunlit leisure time, and children may need to leave for school in the dark.[10]

Critics argue that the energy savings of DST are overstated, and that DST can sometimes increase energy consumption and peak demand. Also, the rise of air conditioning calls older energy models into question. In 2000 when parts of Australia began DST in late winter, overall electricity consumption did not decrease, but the morning peak load increased.[11] The peak-load increase may have come from the Australian experiment's unusually late sunrise times, but currently there is no clear evidence that electricity will be saved by the 2007 U.S. rule change.[12].

Studies of outdoor lighting indicate that while it makes potential crime victims feel safer, it may actually encourage crime,[13] so DST's later sunsets may encourage crime too. The above-mentioned 1975 DOT studies of DST and crime were not conclusive, as they covered only two cities and found reduced crime in only one.

DST inherits the disadvantages of standard time, sometimes making them worse. For example, a sundial is harder to read 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. They "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."[14]

Complications from clock shifts

DST's clock shifts have the obvious disadvantage of complexity. People must remember to change their clocks. Some computer-based systems may require downtime or restarting; ignoring this requirement damaged a German steel facility in 1993.[15] 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, broadcasts, and billing systems is common.

These problems are compounded when the DST rules themselves change, as in the Y2K7 problem. Software developers must test and perhaps modify many applications, and users must remember to install updates.[16]

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.[17] Sleep disruption also correlates with a significant increase in fatal accidents the Monday after a Sunday shift, with the larger increase in autumn.[18]

During an autumn transition from 02:00 to 01:00, the clock will read times from 01:00 to 02:00 twice, leading to confusion in birth certificates and the like.[19]

These problems can be avoided by adopting Coordinated Universal Time (UTC), which is unaffected by DST, but UTC is unsuitable for many applications due to its unfamiliarity. Some clock-shift problems could be avoided by adjusting clocks more gradually—for example, Willett originally suggested weekly 20-minute transitions—but this would add complexity and has never been implemented.

Mnemonic

The mnemonic "spring forward, fall back" (also "spring ahead …" or "spring up …" or "… fall behind") helps people remember how to shift clocks. The pun uses "fall" to mean autumn, a common usage in North American English.

Associated practices

Fire safety officials encourage citizens to use the two annual clock shifts 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.[20]

Computing

Many computer systems can shift their clocks automatically when DST starts and finishes, based on their time zone settings.

Zoneinfo

Most Unix-like systems, including GNU/Linux and Mac OS X, use the zoneinfo database, which maps a name 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, e.g., TZ='America/New_York'.

Some applications have a copy of zoneinfo-derived data which is updated separately. For example, Java releases since 1.4 (2002) have integrated a zoneinfo-derived database into the JRE, located in the lib/zi directory; pre-1.4 Java releases support only a single start and stop rule per setting. Oracle releases since 10g (2004) are similar.

POSIX

Some older or stripped-down Unix-like systems, notably AIX before version 5.2 (2002), support only the TZ values required by POSIX, which specify at most one start and stop rule explicitly in the value, e.g., TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00'.[21] TZ must be changed whenever DST rules change, and the new TZ value applies to all years, mishandling some older time stamps.

Microsoft Windows

Windows Vista supports at most two DST start and stop rules per time zone setting. In a U.S. location observing DST, a single Vista setting would support both post-2006 and 1987–2006 time stamps, while mishandling some older time stamps. Older Microsoft Windows systems usually store only a single start and end rule for each zone, so the same U.S. setting would reliably support only post-2006 time stamps.[22]

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)".[23][24] This caused problems with many applications, including Microsoft Outlook.[25] 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 shift 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.[26]

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.[27][28]

Time zone names typically change when DST is observed. American English replaces "standard time" with "daylight time": 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).

Cultural references

See also

References

  1. ^ Benjamin Franklin (1784-04-26). "Daylight saving". Journal of Paris.
  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?". The Daily Telegraph.
  5. ^ Linda L. Lawson (2001-05-24). "Testimony before the House Science Committee, Energy Subcommittee, concerning daylight saving time and energy conservation". {{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}}: 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. {{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". Working Paper No. W12350. National Bureau of Economic Research. {{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 daylight saving time save energy? Evidence from an Australian experiment" (PDF). CSEM WP 163. University of California Energy Institute. {{cite journal}}: Check date values in: |date= (help); Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
  12. ^ Adrienne Kandel (2007-02-22). "Electricity savings from early daylight saving time" (PDF). CEC-200-2007-001. California Energy Commission. {{cite journal}}: Cite journal requires |journal= (help)
  13. ^ B.A.J. Clark (2003). "Outdoor lighting and crime, part 2: coupled growth" (PDF). Astronomical Society of Victoria. {{cite journal}}: Cite journal requires |journal= (help)
  14. ^ Robertson Davies (1947). "XIX, Sunday". The Diary of Samuel Marchbanks. Clarke, Irwin.
  15. ^ Peter G. Neumann (1994-10-18). "Computer date and time problems". Computer-related risks. Addison-Wesley. ISBN 0-201-55805-X. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  16. ^ 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)
  17. ^ 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. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ 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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  19. ^ "Daylight saving time: rationale and original idea". Retrieved 2007-02-06. {{cite web}}: Unknown parameter |note= ignored (help)
  20. ^ "Turn back your clocks". Consumer Reports. 2006-10. {{cite news}}: Check date values in: |date= (help)
  21. ^ The Open Group (2004). "Other environment variables". IEEE Std 1003.1. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
  22. ^ "Visual Studio and Daylight Saving Time Change". 2007. Retrieved 2007-02-26.
  23. ^ "Australian daylight savings changes for Microsoft products for the year 2006". 2006-08-03. Retrieved 2007-02-16.
  24. ^ "Microsoft products do not reflect Australian daylight saving time changes for the year 2006". 2006-10-30. Retrieved 2007-02-28.
  25. ^ "Aussie Microsoft calendar mess". Office Watch. Vol. 11, no. 5. 2006-01-25.
  26. ^ "Daylight saving time help and support center". 2007-02-16. Retrieved 2007-02-18.
  27. ^ daylight saving time. Merriam-Webster Online Dictionary, accessed July 13, 2006. "called also daylight saving, daylight savings, daylight savings time, daylight time"
  28. ^ 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

External links

Template:Link FA Template:Link FA

Retrieved from "https://en.wikipedia.org/w/index.php?title=Daylight_saving_time&oldid=112975716"