• What is the role of NIST in determining international time?
• Which United States government agencies are responsible for time?
• Is Coordinated Universal Time (UTC) the same as Greenwich Mean Time (GMT)?
• What is the law on time zones?

NIST plays a significant role in determining international time; its atomic clocks contribute to the realization of International Atomic Time (TAI) and Coordinated Universal Time (UTC). Additionally, NIST is among the few primary laboratories worldwide that maintains a precise measurement of the second by realizing its actual duration.

Which United States government agencies are responsible for time?

The National Institute of Standards and Technology (NIST) and the United States Naval Observatory (USNO) can both be considered official sources of time in the United States. Both agencies contribute to the Coordinated Universal Time (UTC) scale maintained by the International Bureau of Weights and Measures (BIPM).

NIST and the USNO maintain separate time scales, but UTC(NIST) and UTC(USNO) are kept in very close agreement, and can be considered equivalent for nearly all purposes. However, the two agencies have different responsibilities and missions. As the nation's national metrology institute, NIST is responsible for providing the ultimate measurement reference for all physical quantities in the United States (not just time and frequency). Calibration and testing laboratories in the United States are often required to show traceability to NIST standards. The USNO is a Department of Defense organization whose primary functions are to provide time for navigation and military purposes. Thus, its primary focus is directed towards meeting the real time requirements of the Departments of Transportation (DOT) and Defense.

Is Coordinated Universal Time (UTC) the same as Greenwich Mean Time (GMT)?

Greenwich Mean Time (GMT) is a 24 hour astronomical time system based on the local time at Greenwich, England. GMT can be considered equivalent to Coordinated Universal Time (UTC) when fractions of a second are not important. However, by international agreement, the term UTC is recommended for all general timekeeping applications, and use of the term GMT is discouraged.

What is the law on time zones?

Time zones are administered by the Department of Transportation. 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.