Time is a motion.
It is a specific, constant quantity of motion that we use to measure all other motions. Just as people use a standard length ruler to measure all other lengths, people use a standard motion to measure all other motions. There is a problem with using the sun's motion as our standard of motion however. That is why Albert Einstein created Relativity, to correct for the error of using a clock based upon the sun's motion. This error would not occur, and Relativity would not be needed, if time was redefined to be the fastest known motion.