S stands for second in mathematics, and S is the abbreviation of second, which is translated into second; H stands for hour, h is the abbreviation of hour, translated into hour; Min stands for minute, which is the abbreviation of minute and translated into minutes.

Definition of second: When cesium-133 atom is in undisturbed ground state at sea level, the radiation frequency Δ ν Cs corresponding to two ultra-fine energy level transitions is expressed in Hz (that is, it is equal to s- 1), and a fixed value of 9 19263 1770 times is taken to define second.

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Under the current international system of units, the second is defined as 9 19263 1770 times the electromagnetic wave period radiated by cesium-133 atom when it transits between two ultra-fine energy levels.

The cesium atom mentioned in this definition must be stationary at absolute zero, and its environment is zero magnetic field. The second defined in this case is equivalent to the second defined in the astronomical almanac.