Daylight saving time (DST) in the states and nations that make use of it now, is a deliberate modification of the framework of numerical values we assign to time to segment the period of our planet Earth's rotational spin, what we call a day. The purpose is to maximize the amount of daylight experienced by the majority of people. To reposition dawn on our artificial clocks, closer to when most arise, and dusk closer to when we go to bed. Humans being evolved physiologically as diurnal (daytime) beings.
Therefore DST is really only practical in the temperate, and possibly the sub-tropical and sub-arctic, zones of the planet. In the tropics around the equator, the period of the day when the sun appears above the horizon remains relatively constant throughout the year. If the Earth had no atmosphere it would be a roughly 50:50 division, 12 hours of daylight and 12 hours of darkness. However, the Earth does have an atmosphere and that atmosphere extends highest above the surface at the equator. Light from the sun is refracted by the atmosphere, bending in towards the surface so that the sun appears visible at dawn and dusk when physically it is actually below the horizon. This makes the daylight portion of the day longer than the dark.
At the poles and the Arctic zones surrounding them we get the opposite extreme. For several weeks both sides of the summer solstice (midpoint) the sun never leaves the sky and for a shorter period, again due to the Earth's refractive atmosphere, around the winter solstice, the sun never rises.
Adjusting the numerical time frame, our hour based artificial clock, by one or two hours at either of these extremes would be pointless in the Arctic regions and probably detrimental in the Tropics. The only possible reason to do so would be to conform to national standards in a nation whose geographical spread included such areas with more politically powerful temperate regions.
In temperate regions we get seasonal variations in the daylight portion of the day. It grows longer in summer and shorter in winter. Daylight saving time adjusts our clocks in summer so we can make the most of the daylight and sets them back to standard in winter so we don't have to be up for too long in the morning before it gets light.
The concept of maximizing the use of the daylight portion of each day has been around since ancient times. Despite have efficient waterclocks, the Romans divided the daylight period into 12 hours no matter what the time of year. Their "hours" varied in duration, shorter in winter and longer in summer; which must have been quite profitable to the waterclock technicians, providing them with an abundance of work adjusting the clocks regularly year round.
The trigger for the modern era's daylight saving time is accredited to a satirical essay by that brilliant thinker, scientist and diplomat, Benjamin Franklin. While in Paris, France in 1784 he noted that many Parisians didn't rise until well after dawn, so wrote his "tongue in cheek" essay on daylight saving and sent it as a letter addressed to the authors of the Journal of Paris. It is still well worth a read, a full copy is available at http://www.webexhibits.org/daylightsaving/franklin3.html for any interested.
The first proposal to actually implement a daylight saving scheme came from William Willett in 1907, two years after he devised the idea. He was a keen golfer and wished to be able to play for longer in the evening, so advocated strongly for England to adopt daylight saving time. Sadly for him, it failed to eventuate before his death in 1915.
The first modern daylight saving scheme started during World War I. Just over a year after Willett's death, Imperial Germany and his (Germany being a Fatherland) allies were having difficulty meeting their needs for coal. They adopted daylight saving time in their controlled areas of Europe to maximize production within the daylight portion of the day to reduce power requirements; considering it easier to change the clocks than the starting times of factory workers.
DST became quite popular in the 20th
Century, but the number of states and nations that still implement it has reduced. Many nations, predominantly within the tropical region, that adopted it have since dropped it; a sensible decision as indicated above.
In those nations that still use DST, it is implemented by reducing the hours officially recognized to 23 for the starting day in spring and increasing them to 25 for the end day in autumn (fall). This is usually managed by manually moving the time registered on clocks forward from 2am to 3am for the start day and from 3am back to 2am again on the end day. This has no actual affect on the day itself, only on how we assign our perceptions of time to it. A purely human conceptual framework that has no fundamental basis in reality.