In 1945 Isador Rabi, a physicist at Columbia University, suggests that a clock could be made using a method (developed by him in the 1930’s) called atomic-beam magnetic resonance could be used as a very accurate time-keeping method.
Using Rabi’s technique, the National Bureau of Standards developed and announces the world’s first atomic watches using the ammonia molecule. The National
International Atomic Time is a very accurate and stable time scale. It is an average of the time kept by many cesium clocks (atomic clocks) all over the world, and has been available since 1955.
True high-precision TAI times can only be determined after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaux. This is done under the auspices of the International Bureau of Weights and Measures. However, atomic clocks are so accurate that only the most precise time computations need to use these corrections, and most time service users use atomic clocks that have been previously referenced to TAI to estimate TAI times for most purposes.
Universal Time (UTC) is the basis for legal time worldwide and follows TAI (see below) exactly except for an integral number of seconds, presently 32. These leap seconds are inserted on the advice of the International Earth Rotation Service (IERS) to ensure that, on average over the years, the Sun is overhead within 0.9 seconds of 12:00:00 UTC on the meridian of Greenwich.
Commercial atomic watches function by sending and receiving radio waves from atomic clocks at places such as the NBS headquarters in Boulder, CO, and adjusting the time to the millisecond.