<P> In the case of the United States, where a one - hour shift occurs at 02: 00 local time, in spring the clock jumps forward from the last instant of 01: 59 standard time to 03: 00 DST and that day has 23 hours, whereas in autumn the clock jumps backward from the last instant of 01: 59 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 to summer time, but instead jumps from 01: 59: 59.9 forward to 03: 00: 00.0 . </P> <P> Clock shifts are usually scheduled near a weekend midnight to lessen disruption to weekday schedules . A one - hour shift is customary . Twenty - minute and two - hour shifts have been used in the past . </P> <P> Coordination strategies differ when adjacent time zones shift clocks . The European Union shifts all zones at the same instant, at 01: 00 Greenwich Mean Time or 02: 00 CET or 03: 00 EET . The result of this procedure is that Eastern European Time is always one hour ahead of Central European Time, at the cost of the shift happening at different local times . In contrast most of North America shifts at 02: 00 local time, so its zones do not shift at the same instant; for example, Mountain Time is temporarily (for one hour) zero hours ahead of Pacific Time, instead of one hour ahead, in the autumn and two hours, instead of one, ahead of Pacific Time in the spring . 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 . In some cases only part of a country shifts; for example, in the U.S., Hawaii and most of Arizona do not observe DST . </P> <P> 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 . Proponents for permanently retaining November as the month for ending DST point to Halloween as a reason to delay the change--to provide extra daylight on October 31 . </P>

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