Atomic clocks Atomic clocks are the most accurate timekeeping devices known to date. Accurate to within a few seconds over many thousands of years, they are used to calibrate other clocks and timekeeping instruments. Atomic clocks have employed other elements, such as hydrogen and rubidium vapor, offering greater stability—in the case of hydrogen clocks—and smaller size, lower power consumption, and thus lower cost (in the case of rubidium clocks).