Top left panel: stellar mass vs. star-formation-weighted ages for the ETG in the cluster (filled red circles) and in the field (filled blue circles). Top right panel: the dependence of the look-back time to form on the galaxy mass. Bottom left panel: rest-frame U− B color vs. age of ETG in both environments. Bottom right panel: formation timescales, τ, of ETG as a function of their stellar mass in both the field and cluster environment. The mean error in stellar age is 0.5 Gyr. Uncertainties in stellar masses and τ are ~0.15 dex and ~0.2 Gyr, respectively.