Why Do The Clocks Go Back Every Year? Daylight Saving Explained

Daylight saving time (DST) goes into effect in March every year. DST is an annual period that sees the time move forward by an hour. Then later in November, the clocks go back by an hour when the DST period ends.

DST begins at 2 a.m. local time on the second Sunday in March, which sees clocks go forward by one hour. So the local time is 3 a.m. and there is one less hour in the day.

Then at 2 a.m. local time on the first Sunday of November, the DST period ends and clocks are turned back by an hour. The local time then becomes 1 a.m., which means there is an extra hour in the day.

In 2021, DST began on March 14 at 2 a.m. local time and ends on November 7 at the same hour.

Why Do the Clocks Go Back Every Year?

The clocks go back every year to set the hours back from DST to standard time (ST). "The transition from DST to ST effectively moves one hour of daylight from the evening to the morning," says the National Institute of Standards and Technology (NIST).

Why Is Daylight Saving Time Observed?

The NIST explains DST creates "more sunlit hours" in the evening during months when the weather is the warmest.

"This change helps keep the hours of daylight coordinated with the time that most people are active. Proponents feel that this saves energy because in the spring and summer months more people may be outside in the evening and not using energy at home.

"There are, however, ongoing debates about how much energy is saved," the NIST adds.

Is Daylight Saving Time Observed Everywhere?

DST was formally introduced in the U.S. in 1918 and is observed in most of the country, apart from a few areas.

Local jurisdictions were historically allowed to decide whether they would observe DST and when they would locally switch to DST if they're observing it.

The Uniform Time Act of 1966 dictates the rules for DST dates and all areas in the country that observe DST use the same transition days.

The NIST notes "that same bill allows states to legislatively decide whether to practice it or not."

The U.S. regions that don't practice DST include Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and most of Arizona (apart from Arizona's Navajo Indian territories, which do observe DST).

Many countries outside the U.S. also observe a period equivalent to DST but the dates for the time changes may differ from the rules in the U.S.

Can Daylight Saving Time Rules Change?

DST is overseen and regulated by the U.S. Department of Transportation.

The rules around the DST date also sometimes change, as they did most recently in 2007. That year the rules changed for the first time in over two decades. Enacted by the Energy Policy Act of 2005, the changes extended the length of DST by about a month "in the interest of reducing energy consumption," the NIST says.

DST is in effect for 238 days (around 65 percent of the year). "Congress retained the right to revert to the prior law should the change prove unpopular or if energy savings are not significant," the NIST explains.

The clock at Grand Central station.
The clock at Grand Central station in New York City pictured on April 24, 2020.