Differences between the quantum mechanical and relativistic concepts of time are explained by using the equivalence principle. The resulting clock model calls for microscopic equations of motion to be formulated in Minkowski space, and for photons to be described as four-dimensional localizations of field. Because the properties of energy are universal the equations have been extended to include galaxies in spite of vast differences in lifetime. The comparison is possible due to a fundamental symmetry based on the conjugate variables energy and time.