The SI unit for time measurements is the second (s). Up until 1960 a second was defined as 1/24×60×60 of a solar day. This definition was vague since better astronomical measurements has shown that the solar day is slowly but steadily becoming longer. During an individual year the solar day is 21 seconds short of 24 hours in September and 29 seconds longer in December due to Earth's elliptical orbit.
The current definition of a second is based on an invariant natural constant, 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.
In the system of natural units (Planck units), the unit of time is called Planck time. It is defined to be the time it takes light to travel one Planck length in vacuum. This means that the unit is defined solely on constants of nature. More exactly tp = (hG/2πc5)1/2 where h is the Planck constant, G is the gravitational constant and c is the speed of light.