Wikipedia contains an article on "International Atomic Time," which provides a general discussion on the topic. For more information, see the NPL page "What is Time?"

The frequency of the atom defines a second and used for international time.

Lawrence Scheinman has written:

'Atomic energy policy in France under the Fourth Republic' -- subject(s): Atomic bomb, Economic aspects of Nuclear energy, Nuclear energy, Politics and government

'The International Atomic Energy Agency and world nuclear order' -- subject(s): Government policy, International Atomic Energy Agency, Nuclear energy, Nuclear nonproliferation

'International law and political crisis' -- subject(s): Addresses, essays, lectures, International law, International relations

'The nonproliferation role of the International Atomic Energy Agency' -- subject(s): International Atomic Energy Agency, Nuclear nonproliferation

'The International Atomic Energy Agency' -- subject(s): International Atomic Energy Agency, Nuclear nonproliferation

Atomic watches receive radio signals from various companies that indicate what the current time is. These signals are released from the International Bureau of Weights and Measures in Paris, ensuring that atomic watches are constantly synchronized with Coordinated Universal Time (UTC).