  We propose that primordial black hole (PBH) binary systems can lead to
standard timers in tracking the evolution of the Universe. Through
gravitational waves from monochromatic PBH binaries, the probability
distribution on major axis and eccentricity from the same redshift is obtained.
By studying the dynamical evolution of PBH binaries from the initial
probability distribution to observed redshifted ones, the redshift-time
calibration can be extracted, which can constrain cosmological models. A
general formalism of the standard timer is further concluded based on the
evolution of statistical distribution in dynamical systems.
