What is Daylight Saving Time

Standards: Daylight Saving Time (DST)

Daylight Saving Time (DST) is a widely used system that adjusts the official local time forward from the official standard time during summer months, usually by one hour.

The Energy Policy Act of 2005 changed the start and end dates of Daylight Saving Time in the United States. As of 2007, DST begins on the second Sunday in March and ends on the first Sunday in November.

The U.S. Department of Energy (DOE) is studying the impact of this DST extension, and the U.S. Congress retains the right to revert to the previous DST schedule if the study shows it does not produce a significant savings of energy.

Use of Daylight Saving Time in the United States, is governed by the Uniform Time Act of 1966. This Act provides a standard for establishing the dates when DST begins and ends in the U.S., while allowing local exemptions from its observance. States wishing to be exempt from DST may pass a law exempting the entire state. The states of Arizona and Hawaii do not observe DST. However, the Navajo Indian Reservation in Arizona does. In addition, American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islands do not observe DST.

Information about the Current Daylight Saving Time (DST) Rules

Daylight Saving Time (DST) rules changed in 2007 for the first time in more than 20 years. The NIST Time Services continue to operate properly under the new DST rules. Let’s take a brief look at what DST is, what the new DST rules are, and how the NIST Time Services have handled the DST changes. Information is also provided for people who may still be having problems with some information technology systems (such as the clock on your computer) since the changes.

What is Daylight Saving Time?
Daylight Saving Time, or DST, is the period of the year when clocks are moved one hour ahead. In the United States, this has the effect of creating more sunlit hours in the evening during months when the weather is the warmest. We advance our clocks ahead one hour at the beginning of DST, and move them back one hour (“spring forward, fall back”) when we return to Standard Time (ST). The transition from ST to DST has the effect of moving one hour of daylight from the morning to the evening. The transition from DST to ST effectively moves one hour of daylight from the evening to the morning.

DST was formally introduced in the United States in 1918. Today, most of the country and its territories observe DST. However, DST is not observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and the state of Arizona (with the exception of the Navajo Indian Reservation, which does observe DST).

Daylight Saving Time and time zones are regulated by the U. S. Department of Transportation, not by NIST. However, as an official timekeeper for the United States, NIST observes all rules regarding DST when it distributes time-of-day information to the public.

What are the current rules for Daylight Saving Time?
The rules for DST changed in 2007 for the first time in more than 20 years. The new changes were enacted by the Energy Policy Act of 2005, which extended the length of DST in the interest of reducing energy consumption. The new rules increased the duration of DST by about one month. DST will now be in effect for 238 days, or about 65% of the year, although Congress retained the right to revert to the prior law should the change prove unpopular or if energy savings are not significant. At present, Daylight Saving Time in the United States

  • begins at 2:00 a.m. on the second Sunday of March and
  • ends at 2:00 a.m. on the first Sunday of November

In 2011, DST is from 2:00 a.m. (local time) on March 13th until 2:00 a.m. (local time) on November 6th.

I set my computer clock to NIST time. How will the DST rules affect me?
There shouldn’t be any problems if your operating system has the latest updates. All NIST time services broadcast Coordinated Universal Time (UTC). Corrections for your local time zone and for Daylight Saving Time are provided by your computer’s operating system. Therefore, it is important that you have the latest software updates for your operating system. For example, if you use Microsoft products, information about DST updates can be found here:

Other operating systems should also have DST updates or patches available. Please check with the provider of your operating system for the latest information. Visit this site for more information and links pertaining to a number of different systems:

The great majority of computer time codes sent by the NIST Internet Time Service (ITS) use the Network Time Protocol (NTP), which contains no information about DST, and relies on your computer’s operating system to determine whether DST or ST is in effect. Therefore, if you use NTP, it is especially important to use an operating system that has been patched to conform to the new DST rules. The NIST time codes sent using the Daytime Protocol of the ITS or sent by telephone using the Automated Computer Time Service (ACTS), do contain DST information and have been modified to conform to the current DST rules. However, the advance notification flags provided by ACTS and by the ITS in daytime format are advisory, and will not override the configuration of your operating system, so it is still important to have the latest operating system updates.

I have a radio controlled clock that receives NIST time. How will the DST rules affect me?
Your radio controlled clock should automatically implement the DST rules. The WWVB broadcast contains information that tells your clock whether DST or ST is currently in effect. NIST always sends this information to agree with the current DST rules, so your clock should change automatically on the day of the change, just as it has in previous years. If the time is wrong after the DST change, make sure that your clock has recently received the time signal. Many radio controlled clocks have a synchronization indicator that will tell you if your clock has recently synchronized. If the clock isn’t receiving the signal, click here for some tips on improving reception. If it has received the signal recently, check to make sure that the clock’s time zone setting is set properly. Also, if you live in an area where DST is not observed (Arizona, for example), you need to disable the automatic DST function on your clock.

Some clocks have the old date rule programmed in and do not use the part of the time code which designates whether or not Daylight Saving Time is in effect. These clocks will switch on the wrong date. The time zone or DST ON/OFF function might need to be switched temporarily so the clock shows the correct hour for the period between the transition dates according to the old rules and the new rules.

I have a non-radio controlled clock that used to automatically correct itself for DST. Will it still work?
The only way to find out is to wait and see, or to contact the manufacturer of the clock. There are a large number of non-radio controlled clocks, marketed under names like “AUTOSET” or “SMARTSET” that have internal firmware that automatically changes the time by one hour on the transition days to and from DST. These clocks do not connect to a NIST time service, and some of them will no longer work properly due to the DST rule change. For example, clocks manufactured prior to the rule change will probably not switch to DST in March, but will wait until April in accordance with the old rule. If you have a clock like this that fails due to the new rules, your only recourse is to adjust the clock manually, or to contact the manufacturer and ask for an upgrade or a replacement.

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