How Does Daylight Saving Time Work?
When Daylight Saving Time (DST) begins, we lose an hour. When it ends, we gain an hour. So how exactly does the DST switch work?
When Daylight Saving Time starts in the spring, we lose an hour of sleep.
©iStockphoto.com/Mypurgatoryyears
What Is Daylight Saving Time?
DST is a seasonal time change measure where clocks are set ahead of standard time during part of the year. As DST starts, the Sun rises and sets later, on the clock, than the day before.
Today, about 40% of countries worldwide have DST to make better use of daylight and conserve energy.
Upcoming DST switches in your city
Spring Forward in Spring
When DST starts in the spring, our clocks are set forward by a certain amount of time, usually by one hour. This means that one hour is skipped, and on the clock, the day of the DST transition has only 23 hours.
Since DST switches usually occur at night to avoid disrupting public life, they snatch away an hour of our usual sleeping time, forcing us to adjust our body clocks. If you set your alarm to the same time as before the clock change, you will sleep an hour less. The good news is that if you work a night shift, you will get away with working one hour less that night.
Example of DST Start
The DST period in the United States begins at 02:00 (2 am) local time, so the hour from 2:00:00 to 2:59:59 does not exist in the night of the switch. It is skipped as clocks spring forward from 1:59:59 standard time to 3:00:00 Daylight Saving Time (see table).
| Local Time | DST or Standard Time? | Comments |
|---|---|---|
| 1:59:58 | Standard | |
| 1:59:59 | Standard | |
| 3:00:00 | DST | DST starts, clocks jump ahead by 1 hour |
| 3:00:01 | DST | |
| 3:00:02 | DST |
Note: the DST period may begin and end at different local times in other countries, but the principle is the same.
Fall Back in Fall
In the fall (autumn), the DST period usually ends, and our clocks are set back to standard time again. In terms of hours on the clock, we gain one hour, so the day of the transition is 25 hours long.
In effect, one hour is repeated as local time jumps from DST back to standard time. Let's say that clocks fall back from 2 to 1 o'clock. This means that the hour between 1 and 2 o'clock happens twice during the night of the switch.
It also means that a time like 01:30 (1:30 am) refers to two different moments, which are one hour apart. So if you're out to meet somebody during that hour—which really lasts two hours—make sure to specify if the meeting is before the switch (first hour) or after it (second hour).
Example of DST End
In the United States, DST always ends at 02:00 (2 am) local time, and clocks are set back to 01:00 (1 am). The table below shows the moment when the time first reaches 1:59:59 and clocks jump back to 1:00:00 standard time and begin ticking towards 2 o'clock for a second time. When the repeated hour is over, local time goes from 1:59:59 to 2:00:00, just like on any other day.
| Local time | DST or standard time? | Comments |
|---|---|---|
| 1:59.58 | DST | |
| 1:59.59 | DST | |
| 1:00:00 | Standard | DST ends, clocks fall back by 1 hour |
| 1:00:01 | Standard | |
| 1:00:02 | Standard |
DST Doesn't Really Make the Evening Longer
It is often said that evenings are longer during DST as the Sun sets one hour later. But that is only half true: On the day after DST starts, the Sun does indeed rise and set at a later time on the clock, creating the illusion of a longer evening.
DST only affects our civil time. It does not alter the Sun's course, the times of sunrise and sunset, or the daylength, which change only gradually as seasons shift during the course of a year.
What DST does is to change the time we use to schedule our daily routines, shifting it in relation to solar time, which is defined by the Sun's course. When we spring forward as DST begins, our clocks show a later time at sunrise, solar noon, and sunset. But, even though days are longer during the summer, that does not mean that these events suddenly occur later when we change our clocks. For example, if the Sun sets at 18:00 (6 pm) on the day before DST starts and at 19:01 (7:01 pm) on the day after, the actual day-to-day difference, in astronomical terms, is one minute.