SI base unit
In the International System of Units (SI), the unit of time is the second (symbol: formula_2). It is a SI base unit, and has been defined since 1967 as "the duration of [cycles] of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". This definition is based on the operation of a caesium atomic clock.These clocks became practical for use as primary reference standards after about 1955, and have been in use ever since.