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, σ. 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 σ<300 km s−1.