John D. Barrow, Cosmology -- The Origin Of The Universe
New York: 1994

ISBN 0465053548
Page 94

This democratic treatment of observers in Einstein’s general theory of relativity means that there is no preferred way of telling time in the universe. Nobody ever measures some absolute phenomenon called “time”; what one measures is the rate of some physical change in the universe. It could be the fall of sand in an egg-timer, the movement of the hands on a clock face, or the dripping of a tap. There are countless changing phenomena that could be used to define the passage of time. For instance, on a cosmic scale, observers around the universe could use the falling temperature of the background radiation to tell time. No one particular measure of change seems to be more fundamental than any other.