  We propose to use relative galaxy ages as a means of constraining
cosmological parameters. By measuring the age difference between two ensembles
of old galaxies at somewhat different redshifts, one could determine the
derivative of redshift with respect to cosmic time, dz/dt. At high redshifts,
z=1-2, this measurement would constrain the equation-of-state of the dark
energy, while at low redshifts, z< 0.2, it would determine the Hubble constant,
H_0. The selected galaxies need to be passively-evolving on a time much longer
than their age difference.
