The content of primary school mathematics teaching mainly includes four aspects. One is number and algebra; The second is graphics and geometry; The third is statistics and probability; The fourth is synthesis and practice. Where hours, minutes and seconds belong to numbers and algebra.

Time unit second is the basic unit of time in the international system of units, with the symbol S, sometimes marked with English abbreviation sec. 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.