Time is a fundamental quantity. but we actually measure time with change in the position of objects. for example one orbit which earth completes is a year. one rotation of earth is a day, one second is defined as 9192631770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium atom. so time is based upon on motion. why do have time as the fundamental quantity is it a convention or is there a reason for having time as a fundamental quantity?