The 'Red Giant Clock' as an indicator for the efficiency of central mixing in horizontal-branch stars

New evolutionary evaluations have been compared with observational data for the globular cluster M5 in order to assess the kind of mixing (semiconvection with and without the inclusion of the breathing pulses or overshooting) which takes place in real stars during the central He-burning phase. It is suggested that the evolutionary time scale on the red giant branch should be used to assess the absolute time scale of evolution on the asymptotic giant branch. On this basis, it is found that ordinary semiconvection without breathing pulses leads to the best match to the observed data for M5.