The basic unit of time is the SECOND, which is a unit of measurement for time. It is the base unit of time in the International System of Units (SI). The SECOND is defined as the duration of 9, 192, 631, 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. It is also equal to 1/60th of a minute, 1/3, 600th of an hour, and 1/86, 400th of a day.