A small number of national metrology laboratories realize the unit of time with the highest accuracy. To do so, they design and build primary frequency standards that produce electric oscillations at a frequency whose relationship to the transition frequency of the atom of caesium 133, which defines the second, is known. In 1997, the best of these primary standards produces the SI second with a relative combined standard uncertainty of 2 parts in 10¹⁵. It is important to note that the definition of the second should be understood as the definition of the unit of proper time: it applies in a small spatial domain which shares the motion of the caesium atom. In a laboratory sufficiently small to allow the effects of the non-uniformity of the gravitational field to be neglected when compared to the uncertainties of the realization of the second, the proper second is obtained after application of the special relativistic correction for the velocity of the atom in the laboratory. It is wrong to correct for the local gravitational field.

Primary frequency standards can also be used for calibration of the frequency of secondary time standards used in national time-service laboratories. These are generally commercial caesium clocks characterized by extreme long-term stability: able to maintain a frequency with a stability better than 1 part in 10¹⁴ over a few months, they constitute very good "time keepers". The relative uncertainty of their frequencies is of the order of 10^–12. Time metrology laboratories also use hydrogen masers with good short-term stability. These instruments are used in all applications which require a stable reference over intervals of less than one day (stability of 1 part in 10¹⁵ at 10 000 s). In their basic form, hydrogen masers are subject to frequency drifts that become apparent when their mean freqencies are compared with that of a caesium clock over a few days. This drift is greatly reduced when the masers are operated in an active mode with a self-servo-controlled cavity. Caesium clocks and hydrogen masers must be operated under carefully controlled environmental conditions.