Daylight Saving Time (DST) and Atomic Clock Accuracy

Daylight Saving Time brings with it the promise of upcoming spring and of longer days ahead. Typically, it begins the 2nd Sunday in March and ends the 1st Sunday in November. Its history is a bit convoluted and not everywhere in the world, or even in the United States, observe it.  This blog will discuss its history, where it is and isn’t observed, and conclude with how atomic clocks are able to stay accurate despite all of this.  Information included in this blog can be found at Time and Date, Web Exhibits, and the National Institute of Standards and Technology.

History of Daylight Saving Time (DST) in the USA

The United States first observed Daylight Saving Time in 1918. The US has observed DST for 103 years between 1918 and 2020. Daylight Saving Time (DST) in the USA starts on the 2nd Sunday in March and ends on the 1st Sunday in November. The current schedule was introduced in 2007 and follows the Energy Policy Act of 2005. According to section 110 of the act, the US Department of Transportation (DOT) governs the use of DST. The law does not affect the rights of the states and territories that choose not to observe DST.

Historically, there were no uniform rules for DST from 1945 to 1966. This caused widespread confusion, especially in transport and broadcasting. The Uniform Time Act of 1966 aligned the switch dates across the USA for the first time. Following the 1973 oil embargo, the US Congress extended the DST period to 10 months in 1974 and 8 months in 1975, in an effort to save energy. After the energy crisis was over in 1976, the DST schedule in the US was revised several times. From 1987 to 2006, the country observed DST for about 7 months each year.

Observance of DST

In the U.S., clocks change at 2:00 a.m. local time. In spring, clocks spring forward from 1:59 a.m. to 3:00 a.m.; in fall, clocks fall back from 1:59 a.m. to 1:00 a.m. In the EU, clocks change at 1:00 a.m. Universal Time.

For the U.S. and its territories, Daylight Saving Time is NOT observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands, the Commonwealth of Northern Mariana Islands, and Arizona. The Navajo Nation participates in the Daylight Saving Time policy, even in Arizona, due to its large size and location in three states.

A quick look at the history of observances in all 7 continents shows the following:

  • Africa: As of 2020 there are 2 countries in Africa that observe DST. There are 16 countries that no longer observe DST and 38 countries that have never observed DST.
  • Antarctica: There are no time zones or countries on this continent.
  • Asia: As of 2020 there are 7 countries that observe DST. There are 25 countries that no longer observe DST and 19 countries that have never observed DST.
  • Australia and Pacific: As of 2020 there are 5 countries in Australia and the Pacific that observe DST. There are 3 countries that no longer observe DST and 11 countries that have never observed DST.
  • Europe: As of 2020 there are 49 countries in Europe that observe DST. There are 5 countries that no longer observe DST and 1 country that never observed DST.
  • North America: As of 2020 there are 8 countries in North America that observe DST. There are 11 countries that no longer observe DST and 20 countries that have never observed DST.
  • South America: As of 2020 there are 2 countries in South America that observe DST. There are 7 countries that no longer observe DST and 5 countries that have never observed DST.

How do Atomic Clocks Stay Accurate During DST?

An atomic clock has an atomic oscillator inside (such as a cesium or rubidium oscillator). A radio-controlled clock has a radio inside, which receives a signal that comes from a place where an atomic clock is located.

In the United States, the signals received by radio-controlled clocks originate from NIST Radio Station WWVB, which is located near Fort Collins, Colorado. WWVB broadcasts on a frequency of 60 kHz. Your radio-controlled clock actually has a miniature radio receiver inside, which is permanently tuned to receive the 60 kHz signal.

At 60 kHz, there isn’t enough bandwidth to carry a voice or any type of audio information. Instead, all that is sent is a code, which consists of a series of binary digits, or bits, which have only two possible values (0 or 1). These bits are generated at WWVB by raising and lowering the power of the signal. They are sent at a very slow rate of 1 bit per second, and it takes a full minute to send a complete time code, or a message that tells the clock the current date and time. When you turn a radio-controlled clock on, it will probably miss the first-time code, so it usually takes more than one minute to set itself (sometimes 5 minutes or longer) depending on the signal quality and the receiver design.

Once your radio-controlled clock has decoded the signal from WWVB, it will synchronize its own clock to the message received by radio. Before it does so, it applies a time zone correction, based on the time zone setting that you supplied. After it has synchronized, it won’t decode the signal from WWVB again for a while. Most clocks only decode the signal once per day, but some do it more often (for example, every 6 hours). Those that decode the signal just once per day usually do it at midnight or in the very early hours of the morning, because the signal is easiest to receive when it is dark at both WWVB and at the site where the clock is located. In between synchronizations, the clocks keep time using their quartz crystal oscillators.

When working properly, radio-controlled clocks always display the correct time, down to the exact second. This means that you should never have to adjust them. During the transition from standard time to daylight saving time (DST) they “spring forward” one hour, and when DST is finished, they “fall back” one hour. If you live in an area that does not observe DST there is likely a toggle switch on your clock to turn the DST option off. If no toggle switch exists, it may be necessary to change the time zone for which your clock sets itself to in order to allow the correct synchronization of time.

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