Thursday, September 16, 2010

How it works

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 atUTC+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.[27] 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.[28] 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. In the past, Australian districts went even further and did 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.[29] In some cases only part of a country shifts; for example, in the U.S., Hawaii and most of Arizona do not observe DST.[30]

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.[28] 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.[31] 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.[32]

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.[33] The time difference between the United Kingdom and mainland Chile may therefore be five hours during the Northern summer, three hours during the Northern winter and four hours a few weeks per year because of mismatch of changing dates.

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 106° 39′ W longitude, slightly west of center of the idealized Mountain Time Zone (105° W), but the time in Saskatchewanis Central Standard Time (90° W) year-round, so Saskatoon is always about 67 minutes ahead of mean solar time.[34] Conversely, northeastIndia and a few other areas skew time zones eastward, in effect observing negative DST.[35] The United Kingdom and Irelandexperimented with year-round DST from 1968 to 1971 but abandoned it because of its unpopularity, particularly in northern regions.[36]

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 165° 24′ W longitude, which is just west of center of the idealized Samoa Time Zone (165° W), but Nome observes Alaska Time (135° W) with DST, so it is slightly more than two hours ahead of the sun in winter and three in summer.[37] Double daylight saving time has been used on occasion; for example, Britain used it during World War II.[28]

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.[38] Only a minority of the world's population uses DST because Asia and Africa generally do not observe it.

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