  Surface temperature distribution of horizontal-branch (HB) stars is very
sensitive to age in old stellar systems, which makes it an attractive age
indicator. In this paper, we present the recent revision of our model
calculations for the HB morphology of globular clusters. The result is more
updated version of our earlier models (Lee, Demarque, & Zinn 1994), which
suggests that the HB morphology is more sensitive (\sim 40%) to age. We also
present our new model calculations on the effect of HB stars in dating old
stellar systems using the H$\beta$ index. Our results indicate that the effect
of HB stars is rather strong, and suggest a possibility for the systematic
difference in age between the globular clusters in the Milky Way Galaxy and
those in giant elliptical galaxies. Finally, we compare these results with the
relative ages estimated from our far-UV dating technique for giant elliptical
galaxies.
