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Frequently asked questions (FAQ)


 What time is it? 

 NIST radio broadcasts 

 NIST time 

 Local time 

 Leap seconds 

 Timekeeping and clocks 

 Times of day 

 Glossary 


Daylight saving time rules, exceptions and legislation.
Time zones, borders, abbreviations and relationship to U.S. postal zip codes.

What is daylight saving time (DST)?
When is daylight saving time (DST)?
Who follows daylight saving time and how is it regulated?
How are time zone borders decided and regulated?
Can I determine a time zone based on a postal zip code?
How are international time zones and DST rules decided?
How are time zones referred to during standard time and daylight saving time?
What are the time zones in the United States?


What is daylight saving time (DST)?

Daylight saving time (DST) is the part of the year when we advance our clocks by one hour, shifting the time of day in relation to where the Sun is above Earth. In other words, during DST the "daylight" begins an hour later in the morning and lasts an hour longer in the evening. 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 California Energy Commission has additional information about DST and links to several studies about its effects on energy consumption.  

When is daylight saving time (DST)?

Daylight saving time (DST) begins each year on the second Sunday in March at 2 a.m. (local time).
Clocks must be moved ahead one hour when DST goes into effect.

The changeover back to standard time (ST) occurs on the first Sunday in November at 2 a.m. (local time).
Clocks must be moved back one hour.

Some people remember which way to move their clocks using the phrase, "spring forward, fall back".

Who follows daylight saving time and how is it regulated?

Most of the United States follows daylight saving time, but a few regions do not. Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and most of Arizona do not observe DST. In Arizona, the Navajo Indian territories do observe DST. Historically, local jurisdictions were allowed to decide when they would locally switch to DST, or not to observe DST at all. The Uniform Time Act of 1966 defined the rules for the dates of DST and all regions that practice DST use the same transition days. However, that same bill allows states to legislatively decide whether to practice it or not. Also, the date rules sometimes change, most recently in 1986 and 2007, extending the length of DST. The Department of Transportation (and not NIST) oversees and regulates DST.

How are time zone borders decided and regulated?

The basis of time zones was to divide Earth into sections of longitude 15 degrees each. That would allow for 24 zones to represent 24 hours in the day (one rotation of Earth). However, in the United States, the borders were usually drawn to follow state lines, or natural landmarks and county lines. There are even a few instances of counties split into two time zones. Time zone boundaries can be legislated at the state and/or county level and also by the U. S. Department of Transportation.

For more details on U.S. time zones, the federal law on time zones is defined in United States Code; Title 15 - Commerce and Trade; Chapter 6 - Weights and Measures and Standard Time; Subchapter IX - Standard Time; Sections 260-267. See United States Law on Standard Time, 15 U.S.C. §6(IX)(260-7).

NIST is an agency of the United States Department of Commerce, and is not involved in the administration of time zones.

Can I determine a time zone based on a postal zip code?

Since U.S. postal zip codes primarily follow county borders, and most U.S. time zone lines are drawn along county borders, an online zip code database may be used to find time zones based on zip codes. A free application that does this is available at: www.zipinfo.com

How are international time zones and DST rules decided?

Much like the United States, countries around the world choose their time zone borders legislatively. Many countries also observe some equivalent of DST, although the name is usually different, and the dates of the time changes may be different from U.S. rules. There is no single body that regulates time zones or daylight saving time around the world. Some countries provide little or no notice when they change their DST status and/or date rules.

Seasons are opposite in the northern and southern hemispheres. So for countries in the southern hemisphere who observe some form of DST, the dates of the time changes occur at opposite times of year from the United States.

How are time zones referred to during standard time and daylight saving time?

Designation of the time of day for specific time zones can be qualified by using the term "standard time" or "daylight time" to indicate which is being followed. For instance, on the United States West Coast, the time of day can be referred to as Pacific standard time (PST) or Pacific daylight time (PDT). Also, for year-round use such as hours-of-operation for a business, the designation can be left off (i.e. Pacific time or PT could be used). This infers "local time" (DST or ST, whichever it is at present).

What are the time zones in the United States?

 Time Zone  Standard time
abbreviation
and UTC offset
Daylight time
abbreviation
and UTC offset
 Notes
       
 Atlantic (AT)  AST -4
 ADT ---
 None of the U.S. territories in AT follow DST
 Eastern (ET)  EST -5
 EDT -4
 
 Central (CT)
 CST -6
 CDT -5
 
 Mountain (MT)
 MST -7
 MDT -6
 Most of Arizona is MST year-round
 Pacific (PT)
 PST -8
 PDT -7
 
 Alaska (AKT)
 AKST -9
 AKDT -8
 
 Hawaii-Aleutian (HT)
 HST -10
 HDT -9
 DST observed in Aleutian Islands, but not Hawaii  
 Samoa (ST)
 SST -11
--- ---
 DST not observed
 Chamorro (ChT)  
 ChST +10
--- ---
 DST not observed