  The UV upturn phenomenon observed in elliptical galaxies is attractive for
its potential value as an age indicator of old stellar systems. We present our
most recent population models for the UV evolution of elliptical galaxies. We
confirm that the dominant UV sources are either metal-poor or metal-rich hot
horizontal-branch (HB) stars in local giant ellipticals, but we also note that
the contribution from post-asymptotic-giant-branch (PAGB) stars overwhelms the
UV spectrum at higher redshifts (look-back times). The model UV spectral energy
distribution (SED) is therefore strongly affected by the current uncertainty of
the mean mass of PAGB stars at higher redshifts. Fortunately, our models
suggest that the far-UV observations at z > 0.35 could produce strong
constraint on the PAGB mass, while observations at could produce strong
constraint on the PAGB mass, while observations at lower redshifts (0.15 < z <
0.30) would still provide constraints on the models on the origin of the UV
upturn. Future observations of ellipticals from the STIS/HST and planned GALEX
space UV facility will provide crucial database required for more concrete
calibration of our UV dating techniques for old stellar systems.
