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Who establishes the correct time in the United States?

The United States National Institute of Standards and Technology (NIST) uses an atomic cesium beam clock to determine the correct time in the United States. The atomic second was adopted as the basic unit of atomic time in 1967, by the thirteenth General Conference of Weights and Measures. An atomic second is defined as 9,192,631,770 oscillations (vibrations) of the radioactive cesium atom. This cesium-beam clock of the NIST is referred to as a "primary clock" because, independently of any other reference, it keeps highly precise time.

Sources: Considine, Glenn D., ed. Van Nostrand's Scientific Encyclopedia, 8th ed., p. 3101; Passport to World Band Radio, p. 70.

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1. Time
   1. How is time measured?
   2. What is meant by Universal Time?
   3. How many time zones are there in the world?
   4. Who establishes the correct time in the United States?
   5. What is military time?
   6. What do the initials a.m. and p.m. mean?
   7. When is Daylight Savings Time observed in the United States?
   8. What is a leap second?
   9. What is the exact length of a calendar year?
   10. Where did the term "grandfather clock" come from?
   11. What were the longest and shortest years on record?
   12. Besides the Julian and Gregorian calendars, what other calendars have been used?
   13. When does leap year occur?
   14. When did January 1 become the first day of the new year?
   15. How were the months of the year named?
   16. Where do the names of the days of the week come from?
   17. Why are the lengths of the seasons not equal?
2. Weights And Measures
   1. What is the metric system of measurement?
   2. What are the U.S. and metric units of measurement for area?
   3. How did the yard as a unit of measurement originate?
   4. What is a bench mark?
   5. How was the length of a meter originally determined?
   6. How are U.S. customary measures converted to metric measures and vice versa?
   7. What is the difference between a short ton, a long ton, and a metric ton?
   8. Why is a nautical mile different from a statute mile?
   9. What are the equivalents for dry and liquid measures?
   10. What are U.S. and British measures for volume and weight of water?

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