What is the Time?
Definition of time
The SI unit of time, the second (s), was defined in 1967 by the 13th General Conference of Weights and Measures in the following terms:
"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."
The World's first Caesium Atomic Frequency Standard
The start of atomic time-keeping, 1955 Essen (right) and Jack Parry with the caesium standard they developed, accurate to one second in 300 years. It is now in the Science Museum
Why we need accurate time
Highly accurate timekeeping is one of the mainstays of modern life.
View the many uses of time and why it has to be so precise (
PDF 35 KB).
The World Time System
This explains how NPL contributes to UTC, the time scale used around the world: The World Time System ( PDF 100 KB)
The Caesium Fountain Primary Frequency Standard
An explanation of the theory and accuracy of atomic clocks and an introduction to the NPL Caesium Fountain
Time scales
The leap second
What are leap seconds and why do we need them? ( PDF 40 KB)
Summer time dates
The start and end dates of Summer Time for the years 2001 to 2011 ( PDF 30 KB)