Yesterday, many users of iOS 7 operating system complained about their clocks being out of time after the operating system automatically adjusted to winter time. So what is summer time, and how is summer time different from winter time?

Daylight Saving Time – also known as DST (Daylight Saving Time) is a type of convention that adjusts the time according to the actual time that it’s light in the day. For countries that apply this convention, from about mid-spring (late March) to early winter (late October, early November), the clock will be adjusted 1 hour faster. The rest of the time, the clock will be reset.

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Why adjust the clock?

When talking about time, people usually understand by two concepts. The first concept is the time in terms of the brightness of the day. With this understanding, the day can be divided into two parts: “daytime”, that is, the time of day, and “night”. The second way to tell time is to divide the date into equal intervals, as most people now understand a day’s day is divided into 24 hours.

However, due to the physical nature (Earth’s axis is offset by an angle of 23.5 degrees), between different seasons, the amount of light in each day also varies. In the summer, the day will be longer than the night, and in the winter it will be the opposite. This phenomenon is more evident at locations near the Earth’s poles.

In the summer, the sun rises earlier, and the days are longer. Photo: National Geographic

Humans have been aware of this phenomenon since time immemorial. Some societies, such as the ancient Romans, divided the time of day by evenly dividing the length of the day, and these periods may vary by season. For example, with their water clocks, during the Winter Solstice (mid-winter) each hour of daylight lasted only 44 minutes now, while during the Summer Solstice (mid-summer) each hour lasted up to 75 minutes long.

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By daylight saving time, clocks are adjusted one hour early for most of the year. Photo: iStockPhoto

Later, the way to divide a day into 24 hours began to be popular and widely used. The feature of this division is that the actual time of each hour is constant between times of the year. However, this division will make the use of daylight less efficient.

In society, the time to work or study is often strictly regulated, according to the convention of a 24-hour day. Meanwhile, the body’s activities often follow the control of light: Wake up and feel healthy when it’s morning, and go to bed when it’s dark. Therefore, if applying winter working time in summer, people will only wake up when the sun has been bright for a few hours, reducing work efficiency.

The fact that people sleep later in the summer will also cause more electricity to be used for lighting, causing energy waste. This has received more attention since the second half of the last century, when energy shortages became a problem for many countries. That is why daylight saving time was proposed, and is officially known as “Daylight Saving Time” (DST).

When is the clock setting time?

Daylight Saving Time is generally applied from the end of March to the end of October, which is more than half a year. However, the application period of each country is different, and not all countries apply this convention.

Map of areas where DST is currently being applied (blue) and not (red and orange). Photo: Wikipedia

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In Asia, only a few countries apply DST such as Iran, Israel, Turkey… Vietnam and big countries like China, Japan or Korea do not apply this convention. DST countries generally run from the last Friday in March, through to the last Friday or Sunday in October.

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In Europe, most countries except Russia, Iceland and Belarus adopt daylight saving time. DST starts at 1:00 a.m. GMT on the last Sunday in March, and ends at 1:00 a.m. on the last Sunday in October.

In the Americas, three countries in North and Central America (Canada, the United States and Mexico) and a few countries in South America have DST. In the United States, there are some states that do not apply this convention. In most other states, clocks are adjusted at 2 a.m. on the second Sunday in March, and reset on the first Sunday in November.

What effect does setting the clock have?

Just through a few examples above, we can see that the clock adjustment time according to DST in the world is very diverse and complex. The application of this convention also has many effects on human health, requires a regulatory process and is also controversial about its application.

Many studies show that adjusting the clock to run early at the end of March affects human health. Specifically, adjusting the time will affect sleep, and it may take a few days to get used to it. A 2008 study also found that, in the first week after adjusting the clock, the number of heart attacks will increase dramatically.

Is DST really energy efficient? Photo: ABC News

Another purpose of DST to save energy is also not really convincing. Although people will sleep earlier in the summer, they still go to work every day, so it still needs energy to turn on the lights during work time. In addition, longer days also cause people to spend more time outside, so gas or electricity to drive cars will cost even more.

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At the time of adopting daylight saving time, people need to adjust their activities to match the new time

Modern computers and phones already have a DST time adjustment feature, but this feature does not always work. Besides the recent iOS 7 bug, Apple also had problems with DST in 2010, when a bug in iOS 4 caused the alarm clock to go off an hour after the time was adjusted. The change of time also causes time-related jobs such as performance hours, flight times, etc. to be changed, reversed and brought about many misunderstandings.

In Vietnam, although daylight saving time is not applied, you can still realize its influence if you follow international events, or work with foreigners. Especially, if you are a football fan, you will easily notice the influence of DST.

During most of the season, football fans will have to stay up later to watch exciting matches

UEFA Champions League matches usually take place at the same time, which is 7:45 am GMT. At the start of the season, because Europe adjusted the time and Vietnam did not, these first matches took place at 1:45 am Vietnam time.

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However, from November to the end of March, which is the majority of a football season, times are adjusted, and UEFA Champions League matches take place at 2:45 am, and will end quite late (nearly 5 am), affecting the living and working schedule of Vietnamese people.

Mr. Minh