Time is one of the seven fundamental quantities in the SI system (International System of Units). The base unit is the second, and the definition is:
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 the basis of all atomic time scales such as UTC and GPS times
Time can be measured with two different tools: the clock and the calender.
A clock is a physical mechanism that counts the passage of time, while a calender organizes time intervals.
