[[Image:DaylightSaving-World-Subdivisions.png|upright=1.67|thumb|alt=World
map. Europe, Russia, most of North America, parts of southern South
America and southern Australia, and a few other places use DST.
Most of equatorial Africa and a few other places near the equator
have never used DST. The rest of the land mass is marked as
formerly using DST.|Although not used by most of the world's
people, daylight saving time is common in high
latitudes.
]]
Daylight saving time (
DST; also
summer time in
British
English—see
Terminology)
is the practice of advancing clocks so that afternoons have more
daylight and mornings have less. Typically
clocks are adjusted forward one hour near the start of spring and
are adjusted backward in autumn. Modern DST was first proposed in
1895 by
George Vernon Hudson, a
New Zealand
entomologist. Many
countries have used it since then;
details vary by
location and change occasionally.
The practice is controversial. Adding daylight to afternoons
benefits retailing, sports, and other activities that exploit
sunlight after
working hours, but
causes problems for farming, evening entertainment and other
occupations tied to the sun. Traffic fatalities are reduced when
there is extra afternoon daylight; its effect on health and crime
is less clear. Although an early goal of DST was to reduce evening
usage of
incandescent
lighting, formerly a primary use of electricity, modern heating
and cooling usage patterns differ greatly, and research about how
DST currently affects energy use is limited and often
contradictory.
DST's occasional clock shifts present other challenges. They
complicate timekeeping, and can disrupt meetings, travel, billing,
recordkeeping, medical devices, heavy equipment, and sleep
patterns. Often, software can adjust computer clocks automatically,
but this can be limited and error-prone, particularly when DST
rules change.
Origin
[[Image:Clepsydra-Diagram-Fancy.jpeg|thumb|upright|alt=A water
clock. A small human figurine holds a pointer to a cylinder marked
by the hours. The cylinder is connected by gears to a water wheel
driven by water that also floats a part that supports the
figurine.|In this ancient
water clock, a
series of gears rotated a cylinder to display hour lengths
appropriate for each day's date.]]
Although not punctual in the modern sense, ancient civilizations
adjusted daily schedules to the sun more flexibly than modern DST
does, often dividing daylight into twelve equal hours regardless of
day length, so that each daylight hour
was longer during summer. For example, Roman
water clocks had different scales for different
months of the year: at Rome's
latitude the
third hour from sunrise,
hora
tertia, started by modern standards at 09:02
solar time and lasted 44 minutes at the winter
solstice, but at the summer solstice it
started at 06:58 and lasted 75 minutes. After ancient times,
equal-length civil hours eventually supplanted unequal, so
civil time no longer varies by season. Unequal
hours are still used in a few traditional settings, such as some
Mount Athos monasteries and some Jewish
ceremonies.
[[Image:Franklin-Benjamin-LOC-head.jpeg|thumb|upright=0.7|left|alt=A
seated older Benjamin Franklin from the waist up, with body facing
to viewer's right but head turned toward the artist. Franklin's
waistcoat is bulging a bit, his expression is inscrutable, and his
hair hangs down to his shoulders.|
Benjamin Franklin suggested firing cannons
at sunrise to waken Parisians.]]
During his
time as an American envoy to France, Benjamin Franklin, author of the proverb,
"Early to bed, and early to rise, makes a man healthy, wealthy and
wise", anonymously published a letter suggesting that Parisians
economize on candles by
rising earlier to use morning sunlight. This 1784
satire proposed taxing
shutters, rationing candles, and waking the
public by ringing church bells and firing cannons at sunrise.
Franklin did not propose DST; like ancient Rome, 18th-century
Europe did not keep precise schedules. However, this soon changed
as
rail and communication networks
came to require a standardization of time unknown in Franklin's
day.
[[Image:G.V.-Hudson-Auckland-Islands-Party.jpeg|thumb|upright|left|alt=Fuzzy
head-and-shoulders photo of a 40-year-old man in a cloth cap and
mustache.|
G.V. Hudson invented modern DST, proposing
it first in 1895.]]
Modern DST was first proposed by the New Zealand
entomologist George Vernon Hudson, whose
shift-work job gave him leisure time to collect
insects, and made him aware of the value of after-hours daylight.
In 1895 he
presented a paper to the Wellington Philosophical Society proposing
a two-hour daylight-saving shift, and after considerable interest
was expressed in Christchurch, New Zealand
he followed up in an 1898 paper.
Many
publications incorrectly credit DST's invention to the prominent
English builder and outdoorsman William
Willett, who independently conceived DST in 1905 during a
pre-breakfast ride, when he observed with dismay how many Londoners
slept through a large part of a summer day.
An avid
golfer, he also disliked cutting short
his round at dusk. His solution was to advance the clock during the
summer months, a proposal he published two years later.
As
described in Politics below,
Willett lobbied unsuccessfully for the proposal in the UK until his
death in 1915, and Germany
, its
World War I allies, and their occupied
zones were the first European nations to use Willett's invention,
starting April 30, 1916, as a way to conserve coal during
wartime. Britain, most of its allies, and many European
neutrals soon followed suit. Russia and a few other countries
waited until the next year; and the United States adopted it in
1918. Since then, the world has seen many enactments, adjustments,
and repeals.
How it works
[[Image:Begin CEST Transparent.png|thumb|upright=0.67|alt=Diagram
of a clock showing a transition from 2:00 to 3:00.|When DST starts
in
central Europe, clocks advance
from 02:00
CET to 03:00
CEST.]][[Image:End CEST
Transparent.png|thumb|upright=0.67|alt=Diagram of a clock showing a
transition from 3:00 to 2:00.|When DST ends in central Europe,
clocks retreat from 03:00 CEST to 02:00 CET. Other regions switch
at different times.]]
In a typical case where a one-hour shift occurs at 02:00 local
time, in spring the clock jumps forward from 02:00
standard time to 03:00 DST and that day has
23 hours, whereas in autumn the clock jumps backward from
02:00 DST to 01:00 standard time, repeating that hour, and that day
has 25 hours. A digital display of local time does not read
02:00 exactly at the shift, but instead jumps from 01:59:59.9
either forward to 03:00:00.0 or backward to 01:00:00.0. In this
example, a location observing
UTC+10 during
standard time is at
UTC+11 during DST;
conversely, a location at
UTC−10
during standard time is at
UTC−9
during DST.
Clock shifts are usually scheduled near a weekend midnight to
lessen disruption to weekday schedules.
A one-hour shift is
customary, but Australia's Lord Howe Island
uses a half-hour shift. Twenty-minute and
two-hour shifts have been used in the past.
Coordination strategies differ when adjacent time zones shift
clocks. The
European Union shifts all
at once, at 01:00
UTC; for example,
Eastern European Time is always one
hour ahead of
Central European
Time. Most of North America shifts at 02:00 local time, so its
zones do not shift at the same time; for example,
Mountain Time can be temporarily either
zero or two hours ahead of
Pacific
Time.
Australian districts
go even further and do not always agree on start and end dates; for
example, in 2008 most DST-observing areas shifted clocks forward on
October 5 but Western
Australia
shifted on
October 26.
Start and end dates vary with location and year. Since 1996
European Summer Time has been
observed from the last Sunday in March to the last Sunday in
October; previously the rules were not uniform across the European
Union. Starting in 2007, most of the United States and Canada
observe DST from the second Sunday in March to the first Sunday in
November, almost two-thirds of the year. 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 now that an
energy-consumption study has been done.
[[Image:Time-differences-brz-us.png||thumb|left|alt=Time graph. The
horizontal axis shows dates in 2008. The vertical axis shows the
UTC offsets of eastern Brazil and eastern U.S. The difference
between the two starts at 3 hours, then goes to 2 hours on February
17 at 24:00 Brazil eastern time, then goes to 1 hour on March 9 at
02:00 U.S. eastern time.|In early 2008 central Brazil was one, two,
or three hours ahead of eastern U.S., depending on the
date.]]
Beginning
and ending dates are the reverse 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 year.
[[Image:2007-02-20 time zones.svg|thumb|upright=1.67|alt=Map of the
time zone boundaries of the world. Generally the borders run
north-south and there are about 24 zones, but there are many
exceptions where the borders follow national boundaries and a few
half-hour or quarter-hour zones exist.|
Time
zones often lie west of their idealized boundaries, resulting
in year-round DST.]]
Western
China, Iceland
, and other
areas skew time zones
westward, in effect observing DST year-round without
complications from clock shifts. For example, Saskatoon
, Saskatchewan
, is at longitude, slightly west of center of the
idealized Mountain Time Zone , but the time in Saskatchewan is Central Standard Time
year-round, so Saskatoon is always about 67 minutes ahead of
mean solar time. Conversely, northeast
India
and a few other areas skew time zones eastward, in
effect observing negative DST. The United Kingdom
and Ireland
experimented with year-round DST from 1968 to 1971
but abandoned it because of its unpopularity, particularly in
northern regions.
Western
France
, Spain
, and other
areas skew time zones and shift clocks, in effect observing DST in
winter with an extra hour in summer. For example, Nome, Alaska
, is at longitude, which is just west of center of
the idealized Samoa Time Zone , but
Nome observes Alaska Time with DST,
so it is slightly more than two hours ahead of the sun in winter
and three in summer. Double daylight saving time has been
used on occasion; for example, Britain used it during
World War II.
DST is generally not observed near the
equator, where sunrise times do not vary enough to
justify it.
Some countries observe it only in some
regions; for example, southern Brazil
observes it
while equatorial Brazil does not. Only a minority of the
world's population uses DST because
Asia and
Africa generally do not observe it.
Benefits and drawbacks
[[Image:William-Willett.jpg|thumb|right|upright|alt=A standing man
in three-piece suit, facing camera. He is about 60 and is bald with
a mustache. His left hand is in his pants pocket, and his right
hand is in front of his chest, holding his pocket watch.|
William Willett independently proposed DST
in 1907 and advocated it tirelessly. ]]
Willett's 1907 proposal argued that DST increases opportunities for
outdoor
leisure activities during afternoon
sunlight hours. The longer days nearer the summer solstice in high
latitudes offer more room to shift daylight from morning to evening
so that early morning daylight is not wasted. DST is commonly not
observed during most of winter, because its mornings are darker:
workers may have no sunlit leisure time, and children may need to
leave for school in the dark.
General agreement about the day's layout confers so many advantages
that a standard DST schedule usually outranks
ad
hoc efforts to get up earlier, even for people who personally
dislike the DST schedule. The advantages of coordination are so
great that many people ignore whether DST is in effect by altering
their nominal work schedules to coordinate with television
broadcasts or daylight.
Energy use
DST's potential to save energy comes primarily from its effects on
residential lighting, which consumes about 3.5% of electricity in
the U.S. and Canada. Delaying the nominal time of sunset and
sunrise reduces the use of artificial light in the evening and
increases it in the morning. As Franklin's 1784 satire pointed out,
lighting costs are reduced if the evening reduction outweighs the
morning increase, as in high-latitude summer when most people wake
up well after sunrise. An early goal of DST was to reduce evening
usage of incandescent lighting, formerly a primary use of
electricity. Although
energy
conservation remains an important goal, energy usage patterns
have greatly changed since then, and recent research is limited and
reports contradictory results. Electricity use is greatly affected
by geography, climate, and economics, making it hard to generalize
from single studies.
- The U.S. Dept. of
Transportation (DOT) concluded in 1975 that DST might reduce
the country's electricity usage by 1% during March and April, but
the National Bureau of
Standards (NBS) reviewed the DOT study in 1976 and found no
significant savings.
- In 2000 when parts of Australia began DST in late winter,
overall electricity consumption did not decrease, but the morning
peak load and prices increased.
- In
Western
Australia
during summer 2006–07, DST increased electricity
consumption during hotter days and decreased it during cooler days,
with consumption rising 0.6% overall.
- Although a 2007 study estimated that introducing DST to Japan
would reduce household lighting energy consumption, a 2007
simulation estimated that DST would increase overall energy use in
Osaka residences by 0.13%, with a 0.02%
decrease due to less lighting more than outweighed by a 0.15%
increase due to extra cooling; neither study examined
non-residential energy use. DST's effect on lighting energy use is
noticeable mainly in residences.
- A 2007 study found that the earlier start to DST that year had
little or no effect on electricity consumption in California.
- A
2007 study estimated that winter daylight saving would prevent a 2%
increase in average daily electricity consumption in Great Britain
.
- A 2008 study examined billing data in Indiana before and after
it adopted DST in 2006, and concluded that DST increased
residential electricity consumption by 1% to 4%, primarily due to
extra afternoon cooling.
- The U.S.
Dept. of Energy
(DOE) concluded in a 2008 report that the 2007 U.S. extension of
DST saved 0.5% of electricity usage during the extended
period.
Several studies have suggested that DST increases motor fuel
consumption. The 2008 DOE report found no significant increase in
motor gasoline consumption due to the 2007 U.S. extension of
DST.
Economic effects
Retailers, sporting goods makers, and other businesses benefit from
extra afternoon sunlight, as it induces customers to shop and to
participate in outdoor afternoon sports. In 1984,
Fortune
magazine estimated that a seven-week extension of DST would yield
an additional $30 million for
7-Eleven
stores, and the
National Golf
Foundation estimated the extension would increase golf industry
revenues $200 million to $300 million. A 1999 study
estimated that DST increases the
revenue of
the
European Union's leisure sector
by about 3%. Conversely, DST can adversely affect farmers and
others whose hours are set by the sun. For example, grain
harvesting is best done after
dew evaporates, so
when field hands arrive and leave earlier in summer their labor is
less valuable. DST also hurts prime-time broadcast ratings and
drive-in and other
theaters.
Changing clocks and DST rules has a direct economic cost, entailing
extra work to support remote meetings, computer applications and
the like. For example, a 2007 North American rule change cost an
estimated $500 million to $1 billion. Although it has
been argued that clock shifts correlate with decreased
economic efficiency, and that in 2000
the daylight-saving effect implied an estimated one-day loss of
$31 billion on U.S. stock exchanges, the estimated numbers
depend on the methodology and the results have been disputed.
Public safety
In 1975 the U.S. DOT conservatively identified a 0.7% reduction in
traffic fatalities during DST, and estimated the real reduction to
be 1.5% to 2%, but the 1976 NBS review of the DOT study found no
differences in traffic fatalities. In 1995 the
Insurance Institute for
Highway Safety estimated a reduction of 1.2%, including a 5%
reduction in crashes fatal to pedestrians. Others have found
similar reductions. Single/Double Summer Time (SDST), a variant
where clocks are one hour ahead of the sun in winter and two in
summer, has been projected to reduce traffic fatalities by 3% to 4%
in the UK, compared to ordinary DST. It is not clear whether
sleep disruption contributes to fatal
accidents immediately after the spring clock shifts. A correlation
between clock shifts and traffic accidents has been observed in
North America and the UK but not in Finland or Sweden. If this
effect exists, it is far smaller than the overall reduction in
traffic fatalities. A 2009 U.S. study found that on Mondays after
the switch to DST, workers sleep an average of 40 minutes less, and
are injured at work more often and more severely.
In the 1970s the U.S.
Law Enforcement
Assistance Administration (LEAA) found a reduction of 10% to
13% in Washington,
D.C.
's violent crime rate during DST. However,
the LEAA did not filter out other factors, and it examined only two
cities and found crime reductions only in one and only in some
crime categories; the DOT decided it was "impossible to conclude
with any confidence that comparable benefits would be found
nationwide". Outdoor lighting has a marginal and sometimes even
contradictory influence on crime and fear of crime.
In several countries,
fire safety
officials encourage citizens to use the two annual clock shifts as
reminders to replace batteries in
smoke and
carbon monoxide detectors,
particularly in autumn, just before the
heating
and candle season causes an increase in home fires. Similar
twice-yearly tasks include reviewing and practicing fire escape and
family disaster plans, inspecting vehicle lights, checking storage
areas for hazardous materials, reprogramming
thermostats, and seasonal
vaccinations. Locations without DST can instead
use the first days of spring and autumn as reminders.
Health
[[Image:Greenwich GB DaylightChart.png|upright=1.67|thumb|alt=Graph
of sunrise and sunset times for 2007. The horizontal axis is the
date; the vertical axis is the times of sunset and sunrise. There
is a bulge in the center during summer, when sunrise is early and
sunset late. There are step functions in spring and fall, when DST
starts and stops.|
Clock shifts affecting apparent sunrise and
sunset times at Greenwich
in 2007.]]
DST has mixed effects on health. In societies with fixed work
schedules it provides more afternoon sunlight for outdoor exercise.
It alters sunlight exposure; whether this is beneficial depends on
one's location and daily schedule, as sunlight triggers vitamin D
synthesis in the skin, but overexposure can lead to
skin cancer. Sunlight strongly influences
seasonal affective
disorder. DST may help in
depression by causing individuals to rise
earlier, but some argue the reverse. The
Retinitis Pigmentosa Foundation
Fighting Blindness, chaired by blind sports magnate
Gordon Gund, successfully lobbied in 1985 and
2005 for U.S. DST extensions, but DST can hurt
night blindness sufferers.
Clock shifts disrupt sleep and reduce its efficiency. Effects on
seasonal adaptation of the
circadian
rhythm can be severe and last for weeks. A 2008 study found
that although male
suicide rates rise in the
weeks after the spring transition, the relationship weakened
greatly after adjusting for season. A 2008 Swedish study found that
heart attacks were significantly more
common the first three weekdays after the spring transition, and
significantly less common the first weekday after the autumn
transition.
The government of Kazakhstan
cited health complications due to clock shifts as a
reason for abolishing DST in 2005.
Complexity
DST's clock shifts have the obvious disadvantage of complexity.
People must remember to change their clocks; this consumes time,
particularly for mechanical clocks that cannot be moved backward
safely. As more devices contain clocks, more time is spent changing
them. People who work across
time zone
boundaries need to keep track of multiple DST rules, as not all
locations observe DST or observe it the same way. The length of the
day becomes variable. Disruption to meetings, travel, broadcasts,
billing systems, and records management is common, and can be
expensive. During an autumn transition from 02:00 to 01:00, a clock
reads times from 01:00:00.0 through 01:59:59.9 twice, possibly
leading to confusion.
[[Image:Willett memorial.JPG|thumb|upright|alt=A standing stone in
a grassy field surrounded by trees. The stone contains a vertical
sundial centered on 1 o'clock, and is inscribed "HORAS NON NUMERO
NISI ÆSTIVAS" and "SUMMER TIME ACT 1925".|The
William Willett Memorial Sundial is always
on DST.]]
Some computer-based systems require downtime or restarting when
clocks shift; ignoring this requirement damaged a German steel
facility in 1993. Medical devices may generate adverse events that
could harm patients, without being obvious to clinicians
responsible for care. These problems are compounded when the DST
rules themselves change, as in the
Year 2007 problem. Software developers
must test and perhaps modify many programs, and users must install
updates and restart applications. Consumers must update devices
such as programmable thermostats with the correct DST rules, or
manually adjust the devices' clocks.
Some clock-shift problems could be avoided by adjusting clocks
continuously or at least more gradually—for example, Willett at
first suggested weekly 20-minute transitions—but this would add
complexity and has never been implemented.
DST inherits and can magnify the disadvantages of
standard time. For example, when reading a
sundial, one must compensate for it along
with time zone and natural discrepancies. Also, sun-exposure rules
like avoiding the sun within two hours of noon become less accurate
when DST is in effect.
Politics
Daylight saving has caused controversy since it began.See
Further reading for full
citations to these books:
- Winston Churchill argued that
it enlarges "the opportunities for the pursuit of health and
happiness among the millions of people who live in this country".
Robertson Davies, however, detected
"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", and wags have dubbed it
"Daylight Slaving Time". Historically, retailing, sports and
tourism interests have favored daylight saving, while agricultural
and evening entertainment interests have opposed it, and its
initial adoption has been prompted by energy crisis and war.
The fate of Willett's 1907 proposal illustrates several political
issues involved.
The proposal attracted many supporters,
including Balfour, Churchill,
Lloyd George, MacDonald, Edward VII (who used
half-hour DST at Sandringham), the
managing director of Harrods
, and the manager of the National Bank.
However, the opposition was stronger: it included Prime Minister
Asquith,
Christie (the
Astronomer Royal),
George Darwin,
Napier
Shaw (director of the
Meteorological Office), many
agricultural organizations, and theater owners.
After many hearings
the proposal was narrowly defeated in a Parliament
committee vote in 1909. Willett's allies
introduced similar bills every year from 1911 through 1914, to no
avail. The U.S. was even more skeptical:
Andrew Peters introduced a DST bill to
the
U.S.
House of
Representatives in May 1909, but it soon died in
committee.
[[Image:Victory-Cigar-Congress-Passes-DST.jpeg|thumb|upright|alt=Poster
titled "VICTORY! CONGRESS PASSES DAYLIGHT SAVING BILL" showing
Uncle Sam turning a clock to daylight saving time as a clock-headed
figure throws his hat in the air. The clock face of the figure
reads "ONE HOUR OF EXTRA DAYLIGHT". The bottom caption says "Get
Your Hoe Ready!"|Retailers generally favor DST.
United Cigar Stores hailed a 1918 DST
bill.]]
World War I changed the political
equation, as DST was promoted as a way to alleviate hardships from
wartime coal shortages and air raid blackouts. After Germany led
the way, the United Kingdom first used DST on May 21, 1916.
U.S.
retailing and manufacturing interests led by Pittsburgh
industrialist Robert Garland soon began lobbying
for DST, but were opposed by railroads. The U.S.'s 1917
entry to the war overcame objections, and DST was established in
1918.
The war's end swung the pendulum back. Farmers continued to dislike
DST, and many countries repealed it after the war. Britain was an
exception: it retained DST nationwide but over the years adjusted
transition dates for several reasons, including special rules
during the 1920s and 1930s to avoid clock shifts on Easter
mornings. The U.S. was more typical: Congress repealed DST after
1919. President
Woodrow Wilson, like
Willett an avid golfer, vetoed the repeal twice but his second veto
was overridden. Only a few U.S. cities retained DST locally
thereafter, including New York so that its financial exchanges
could maintain an hour of
arbitrage
trading with London, and Chicago and Cleveland to keep pace with
New York. Wilson's successor
Warren
G. Harding opposed DST as a
"deception".
Reasoning that people should instead get up
and go to work earlier in the summer, he ordered District of
Columbia
federal employees to start work at 08:00 rather
than 09:00 during summer 1922. Many businesses followed suit
though many others did not; the experiment was not repeated.
Since Willett's day the world has seen many enactments,
adjustments, and repeals of DST, with similar politics involved.
The
history of time
in the United States includes DST during both world wars, but
no standardization of peacetime DST until 1966. In the mid-1980s,
Clorox (parent of
Kingsford Charcoal) and
7-Eleven provided the primary funding for the
Daylight Saving Time Coalition behind the 1987 extension to U.S.
DST, and
both Idaho
senators voted for it based on the
premise that during DST fast-food restaurants sell more French
fries, which are made from Idaho potatoes; in 2005, the Sporting Goods
Manufacturers Association and the National Association
of Convenience Stores successfully lobbied for the 2007
extension to U.S. DST.
After a three-year trial, 55% of Western
Australians
voted against DST in 2009, with urban areas split
evenly and rural areas strongly opposed; it was the fourth
consecutive referendum since 1975 to reject DST. In the UK
the sport and leisure industry supports a proposal to observe
SDST's additional hour year-round.
Terminology
Although
daylight saving time is considered to be correct,
daylight savings time is commonly used. The form
daylight saving time uses the
present participle saving as an
adjective, as in
labor saving device; the first two words
are sometimes hyphenated, as in
daylight-saving time. The
common variants
daylight savings time and
daylight
savings use
savings by analogy to
savings account.
Daylight time
is also common. Willett's 1907 proposal used the term
daylight
saving, but by 1911 the term
summer time replaced
daylight saving time in draft legislation in Britain, and
continental Europe uses similar phrases, such as
Sommerzeit in Germany and
l'heure d'été in
France.
The name of local time typically changes when DST is observed.
American English replaces
standard with
daylight: for example,
Pacific Standard Time
(
PST) becomes
Pacific Daylight Time
(
PDT).
British English
typically inserts
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).
The American English
mnemonic "
spring forward,
fall
back" (also "spring ahead ...", "spring up ...", and
"... fall behind") helps people remember which direction to
shift clocks.
Much of North America now advances clocks
before the vernal equinox, so the
mnemonic disagrees with the astronomical definition of spring, but
a proposed substitute "March forward ..." works only in the
northern
hemisphere
, and is less robust against future rule
changes.
Computing
[[Image:Daylightsavings.jpg|thumb|upright|alt=Strong man in sandals
and with shaggy hair, facing away from artist, grabbing a hand of a
clock bigger than he is and forcing it backwards. The clock uses
Roman numerals and the man is dressed in stripped-down Roman
gladiator style. The text says "You can't stop time... But you can
turn it back one hour at 2 a.m. on Oct. 28 when daylight-saving
time ends and standard time begins."|A 2001 U.S.
public service advertisement
reminded people to adjust clocks manually.]]
Many computer-based systems can adjust automatically when DST
starts and finishes, based on their
time zone settings.
However, changes to DST rules causes problems in existing
installations. For example, the
2007
change to DST rules in North America required many computer
systems to be upgraded, with the greatest impact on email and
calendaring programs like
Microsoft
Outlook; the upgrades consumed a significant effort by
corporate
information
technologists. Some applications standardize on UTC to avoid
problems with clock shifts and time zone differences.
Two computer-based implementations in wide use today are zoneinfo
and Microsoft Windows.
Zoneinfo
The
zoneinfo database maps a name to the
named location's historical and predicted clock shifts. This
database is used by many computer software systems, including most
Unix-like operating systems,
Java, and
Oracle;
HP's "tztab" database is similar but
incompatible. When temporal authorities change DST rules, zoneinfo
updates are installed as part of ordinary system maintenance. In
Unix-like systems the TZ
environment variable specifies the
location name, as in
TZ='America/New_York'
.
Older or stripped-down systems may support only the TZ values
required by
POSIX, which specify at most one
start and end rule explicitly in the value. For example,
TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00'
specifies time
for eastern North America starting in 2007. TZ must be changed
whenever DST rules change, and the new TZ value applies to all
years, mishandling some older time stamps.
Microsoft Windows
As with zoneinfo, a user of
Microsoft
Windows configures DST by specifying the name of a location,
and the operating system then consults a table of rule sets that
must be updated when DST rules change. Procedures for specifying
the name and updating the table vary with release. Updates are not
issued for older versions of Microsoft Windows.
Windows Vista supports at most two start and
end rules per time zone setting. In a Canadian location observing
DST, a single Vista setting supports both 1987–2006 and post-2006
time stamps, but mishandles some older time stamps. Older Microsoft
Windows systems usually store only a single start and end rule for
each zone, so that the same Canadian setting reliably supports only
post-2006 time stamps.
These limitations have caused problems.
For example, before
2005, DST in Israel
varied each
year and was skipped some years. Windows 95 used rules correct for 1995 only,
causing problems in later years. In
Windows
98 Microsoft gave up and marked Israel as not having DST,
forcing Israeli users to shift their computer clocks manually twice
a year. The 2005
Israeli
Daylight Saving Law established predictable rules using the
Jewish calendar but Windows zone files cannot represent the rules'
dates in a year-independent way. Partial workarounds, which
mishandle older time stamps, include manually switching zone files
every year and a Microsoft tool that switches zones
automatically.
Microsoft Windows keeps the system
real-time clock in local time. This causes
several problems, including compatibility when
multi booting with operating systems that set the
clock to UTC, and double-adjusting the clock when multi booting
different Windows versions, such as with a rescue boot disk. In
2008 Microsoft hinted that future versions of Windows will
partially support a
Windows
registry entry
RealTimeIsUniversal that had been
introduced many years earlier, when
Windows
NT supported
RISC machines with UTC clocks,
but had not been maintained since.
References
[[Image:Franklin-Benjamin-Journal-de-Paris-1784.jpg|thumb|upright|alt=Yellowed
magazine cover containing mostly print that is too small to read.
Near the top is "JOURNAL DE PARIS."|
Franklin's 1784 letter about daylight had
neither title nor
byline.]][[Image:Waste-of-Daylight-19-cover.jpg|thumb|upright|alt=Pamphlet
cover showing a large British flag in red, white, and dark blue,
with the large title "THE WASTE OF DAYLIGHT", an unreadable
subtitle, and "WILLIAM WILLETT" near the bottom.|
William Willett's pamphlet promoting DST
went through nineteen editions.]]
- Its first publication was in the journal's "Économie" section.
The revised English version (retrieved on 2009-02-13) is
commonly called "An Economical Project", a title that is not
Franklin's; see
- Willett pamphlet: * *
- An earlier version is in:
- Lay summary – New York Times
(2008-03-08).
- Effect on those whose hours are set by the sun: * *
- Data supporting Coren's half of this exchange are in:
- Clock shifts and accidents: * * * *
- Clock shifts as safety reminders: * * * *
- DST and circadian rhythm: * *
- Daylight saving time and its variants: * * * *
Further reading
- The British version, focusing on the UK, is
External links
- * Saving Time, Saving Energy—North American
viewpoint
- * Summer Time—European viewpoint