Since 1967 , the second has been defined to be : the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom .
As a result , in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units ( SI ) as 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 .