Daylight Saving Time begins for most of the United States at 2:00 a.m. on the first Sunday of April. Time reverts to standard time at 2:00 a.m. on the last Sunday of October. In the U.S., each time zone switches at a different time.

In the European Union, Summer Time begins and ends at 1:00 a.m. Universal Time (Greenwich Mean Time). It begins the last Sunday in March and ends the last Sunday in October. In the EU, all time zones change at the same moment.

One of the major reasons given for observing DST is energy conservation. Theoretically, the amount of residential electricity needed in the evening hours is dependent both on when the sun sets and when people go to bed. Because people tend to observe the same bedtime year-round, by artificially moving sunset one hour later, the amount of energy used is theoretically reduced. United States Department of Transportation studies showed that DST reduces the country's electricity usage by one percent during each day DST is in effect.

Part of the reason that it is normally observed in the late spring, summer, and early autumn is because during the winter months the amount of energy saved by moving sunset one hour later is negated by the increased need for morning lighting by moving sunrise by the same amount. During the summer most people would wake up after the sun rises, regardless of whether daylight saving time is in effect or not, so there is no increased need for morning lighting to offset the afternoon energy savings.

Another perceived benefit of DST is increased opportunities for outdoor activities. Most people plan outdoor activities during the increased hours of sunlight. Other benefits cited include prevention of traffic injuries (by allowing more people to return home from work or school in daylight), and crime reduction (by reducing people's risk of being targets of crimes that are more common in dark areas).

When the U.S. went on extended DST in 1974 and 1975 in response to the 1973 energy crisis, Department of Transportation studies found that observing DST in March and April saved 10,000 barrels of oil a day, and prevented about 2,000 traffic injuries and 50 fatalities saving about U.S. $28 million in traffic costs. (Stats from this article).

DST is not universally accepted; many localities do not observe it. Opponents claim that there is not enough benefit to justify needing to adjust clocks twice every year. The disruption in sleep patterns associated with setting clocks either forward or backward correlates with a spike in the number of severe auto accidents,^[1] as well as lost productivity as sleep-disrupted workers adjust to the schedule change. It is also noted that much effort is spent reminding everyone twice a year of the change, and thousands are inconvenienced by showing up at the wrong time when they forget.

There is also a question whether the savings in lighting costs justifies the increase in summertime air conditioning costs. While most people use more sunlight under DST, most people also experience more heat, which prompts many people to turn on the air conditioner during the warmer afternoon hours. When air conditioning was not widely available, the change did save energy; however, air conditioning is much more widespread now than it was several decades ago. Air conditioning often uses more energy than artificial lighting. It was for this reason that Arizona rejected DST and opted to stay on standard time all year.

It is also speculated that one of the benefits, more afternoon sun, would also actually increase energy consumption as people get into their cars to enjoy more time for shopping and the like.

No formal studies have been performed, but an enormous amount of time has been spent by software developers to deal with the fact that 2400 hours past 2pm is not necessarily 2pm 100 days later.

For example, during a North American time change, an autumn night where clocks are reset from 3 AM summer to 2 AM winter time, times between 2AM and 3AM will occur twice, causing confusion in transport schedules, payment systems, etc.

Some studies do show that changing the clock increases the traffic accident rate. Following the spring shift to Daylight Saving Time (when one hour of sleep is lost) there is a measurable increase in the number of traffic accidents that result in fatalities. Some suspect that the change in lighting conditions confuses drivers in their regular commutes. Others speculate that it relates more to the amount of sleep a person gets. Still others suspect that people hurry because they have looked at an incorrect clock and discover that they are late. And a few even suggest that the traffic accidents are caused by people trying to reset the clocks in their cars while driving.

Some campaigners in Britain would like the country to stay on British Summer Time (BST) all year round, or in other words, adopt Central European Time and abolish BST. Alternatively, some would like Britain to adopt Central European Time and jump forward another hour during the summer (adopting a Single/Double Summer Time from Britain's perspective). This would make winter evenings longer, thereby reducing traffic accidents and cases of seasonal affective disorder. Opponents point to the longer hours of darkness on winter mornings, especially in Scotland, which might well cause an increase in road accidents. It has even been suggested that Scotland should be placed on a different time zone from the rest of the UK, which, unlikely though it may sound, would be possible as the UK Parliament could legislate to put the UK forward an hour, and then the Scottish Parliament could put Scotland back onto GMT.

DST is particularly unpopular among people working in agriculture because the animals do not observe it, and thus the people are placed out of synchronization with the rest of the community, including school times, broadcast schedules, and the like."