  We propose that physical time is based on counting the oscillations of wave
functions. The discrete counting of the ticks of these clocks does not depend
on the metric frame. It remains well defined for the beginning epochs of the
universe. The photon clock counts the oscillations of electromagnetic plane
waves in the cosmic reference frame. It can be gauged with clocks for other
massless or massive particles. These equivalent clocks form a clock system
which defines a common universal continuous physical time. For the photon clock
this physical time coincides with conformal time. The universe is eternal
towards the past for typical models of inflationary cosmology -- the photon
clock ticks an infinite number of times. We compare the photon clock system to
geodesic physical time, which is a frame invariant generalization of proper
time.
