  We present a detailed stellar population analysis of 27 massive elliptical
galaxies within 4 very rich clusters at redshift z~0.2. We obtained accurate
estimates of the mean luminosity-weighted ages and relative abundances of CN,
Mg and Fe as functions of the galaxy velocity dispersion, sigma. Our results
are compatible with a scenario in which the stellar populations of massive
elliptical galaxies, independently of their environment and mass, had formation
timescales shorter than ~1 Gyr. This result implies that massive elliptical
galaxies have evolved passively since, at least, as long ago as z~2. For a
given galaxy mass the duration of star formation is shorter in those galaxies
belonging to more dense environments. Finally, we show that the abundance
ratios [CN/Fe] and [Mg/Fe] are the key "chemical clocks" to infer the star
formation history timescales in ellipticals. In particular, [Mg/Fe] provides an
upper limit for those formation timescales, while [CN/Fe] apperars to be the
most suitable parameter to resolve them in elliptical galaxies with sigma<300
km/s.
